Do not take legacy property into account if there's non-legacy value.

Moves the things around to be able to override the config with the URL
params specified in the hash part of the location URI to which the app
is navigating to.

* feat: Whitelists the gatherStats option.

* doc: Replaces obsolete disableStats option with gatherStats.

They will be stored in redux and the PageReloadOverlay will be displayed.

Note that this commit also introduces a subtle (and yet important!) change:
the location URL is now always set, regardless of the configuration loading or
not. This is needed in order for the retry logic to pick it up.

* Removes unused config logic.

* Whitelists config options that can be overridden using the URL.

* Recorder login with credentials, not supported by externalconnect.

Jibri uses xmpp credentials to login, which is not supported by externalconnect, so we want to skip it till that is supported.

* Whitelist only config.js

* Extracts whitelisting in separate function.

* doc: document disableH264 option

* [RN] Override disableH264 config option

We never want it disabled on mobile.

ESLint 4.8.0 discovers a lot of error related to formatting. While I
tried to fix as many of them as possible, a portion of them actually go
against our coding style. In such a case, I've disabled the indent rule
which effectively leaves it as it was before ESLint 4.8.0.

Additionally, remove jshint because it's becoming a nuisance with its
lack of understanding of ES2015+.

* Javadoc introduced @code as a replacement of <code> and <tt> which is
  better aligned with other javadoc tags such as @link. Use it in the
  Java source code. If we switch to Kotlin, then we'll definitely use
  Markdown.

* There are more uses of @code in the JavaScript source code than <tt>
  so use @code for the sake of consistency. Eventually, I'd rather we
  switch to Markdown because it's easier on my eyes.

* Xcode is plain confused by @code and @link. The Internet says that
  Xcode supports the backquote character to denote the beginning and end
  of a string of characters which should be formatted for display as
  code but it doesn't work for me. <tt> is not rendered at all. So use
  the backquote which is rendered itself. Hopefully, if we switch to
  Markdown, then it'll be common between JavaScript and Objective-C
  source code.

The error is stored in the redux store in base/config so other components can
consult it. It is also broadcasted as a new event in the external API for the
SDK.

This patch loads the config later than we used to, that is, only once we
know the room the user is about to join.

Due to architectural limitations in lib-jitsi-meet, it needs to be
initialized with a configuration in order to properly function. This is
unfortunate because we need to create a video track in the welcome page,
but don't know the room (hence no config) yet. In order to circumvent
this problem an empty configuration is used, which is later swapped with
the appropriate one, once loaded.

Some interesting side-effects of this change are a perceived speed
increase when the app starts or a conference is hangup. They are both
due to the fact that no config needs to be fetched from a remote server
in those cases.

The global.performance.now polyfill is removed since it's polyfilled by
react-native-callstats.

Iterate over objects and copy over primitives and arrays
instead of using _.merge, as merge will not replace a config
entry completely. For arrays in a target object, the arrays
will have its indices replaced. This means if a source array
is empty, the target array will be left alone. Similarly,
if the target array is longer than a source array, there
will be indices not touched in the target array.

Web's ExternalAPI accepts an object with properties as one of its
constructor arguments and from which it generated a URL. Mobile's
JitsiMeetView.loadURLObject is supposed to accept pretty much the same
object.

navigator.product is read-only, so we don't run into the risk of it being
changed.

This makes it possible for the following URL params to work:

config.p2p.enabled=true&config.p2p.preferH264=true

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.

Make 'Add jwt module to react' work on mobile.

The config object defined by lib-jitsi-meet is not used by
lib-jitsi-meet only, jitsi-meet respects its values as well.
Moreover, jitsi-meet defined classes and/or functions which manipulate
that config object. Consequently, it makes sense to move the config
object and the associated classes and functions in a dedicated feature.