"Middleware" redux state changes, not actions.

Adds base/sounds feature which allows other features to register a sound
source under specified id. A new SoundsCollection component will then
render corresponding HTMLAudioElement for each such sound. Once "setRef"
callback is called by the HTMLAudioElement, this element will be added
to the Redux store. When that happens sound can be played through the
new 'playSound' action which will call play() method on the stored
HTMLAudioElement instance.

[RN] Fix PR feedbacks, write persistency docs

On web Conference is pretty much all there is, but on mobile we have the welcome
page and the blank page. If we fail to load config.js, for example we will still
be in the welcome page *and* we want to show an error overlay.

Prepares for the latest eslint & flow-related npm packages which cause
these errors.

e09949be9f introduced the dark theme globally, but
it only applies to web, so move the logic to App.web.js

For example, while config.js and other files are being loaded before the
navigation to Conference is feasible.

Deep/universal linking now utilizes loadURL (when possible). But loadURL
is imperative in the native source code while its JavaScript counterpart
i.e. React App Component prop url is declarative. So there's the
following bug: open a URL, leave the conference (by tapping the hangup
button, for example), and then opening the same URL actually leaves you
on the Welcome page (if enabled; otherwise, a black screen).

The implementation has a flow though: opening the same URL twice in a
row without an intervening leave will leave the first opening and join
the new opening. Which can be improved by not leaving and joining if the
conference is joined, joining, an not leaving. But that can be done
separately as an improvement independent of the current implementation
details.

As https://facebook.github.io/react/docs/typechecking-with-proptypes.html
says, React.PropTypes have moved into the npm package prop-types since
React v15.5. I've already failed to update certain devDependencies
because they mandate the use of prop-types so I'd rather we (gradually
at least) move to prop-types rather than face a lot of work later on.

Introduces loadURLObject in JitsiMeetView on Android and iOS which
accepts a Bundle and NSDictionary, respectively, similar in structure to
the JS object accepted by the constructor of Web's ExternalAPI. At this
time, only the property url of the bundle/dictionary is supported.
However, it allows the public API of loadURLObject to be consumed. The
property url will be made optional in the future and other properties
will be supported from which a URL will be constructed.

React (pun intended) to prop changes, that is, load the new specified URL.

In addition, fix a hidden bug in loading the initial URL from the linking
module: we prefer a prop to the URL the app was launched with, in case somehow
both are specified. We (the Jitsi Meet app) are not going to run into this
corner case, but let's be defensive just in case.

Also expose this in the native SDKs.

It's obsolete now, since config is handled in Redux. Also add a "defaultUrl"
prop so emdedding applications can select what the default base URL is.

The counterpart of the external API in the Jitsi Meet Web app uses the
search URL param jwt to heuristically detect that the Web app is very
likely embedded (as an iframe) and, consequently, needs to forcefully
enable itself. It was looking at whether there was a JSON Web Token
(JWT) but that logic got broken when the JWT support was rewritten
because the check started happening before the search URL param jwt was
parsed.

There were getDomain, setDomain, SET_DOMAIN, setRoomURL, SET_ROOM_URL
which together were repeating one and the same information and in the
case of the 'room URL' abstraction was not 100% accurate because it
would exist even when there was no room. Replace them all with a
'location URL' abstraction which exists with or without a room.

Then the 'room URL' abstraction was not used in (mobile) feature
share-room. Use the 'location URL' there now.

Finally, removes source code duplication in supporting the Web
application context root.

* fix(react/participant): store display name in redux

* feat(remotecontrol): Add option to display the authorization dialog in meet

* feat(remotecontrol): Enable ESLint and Flow

Adds translate function with default namespaces and options.

On RN we don't use the global APP object, so don't save the store there unless
it's defined, which is the case in the current web version. Also, check for
undefined explicitly, since a "if (!APP)" check will throw a ReferenceError.

The mobile app remembers the domain which hosted the last conference. If
the user specified a full URL first and specified a room name only the
second time, it was not obvious that the second conference would be
hosted on the domain of the first conference.

The files react/index.native.js and react/index.web.js ended up having
very similar source code related to initializing the Redux store. Remove
the duplication.

Additionally, I always wanted the App React Component to be consumed
without the need to provide a Redux store to it.