- Remove network-activity "feature"
    - It wasn't in use
    - It relied on internal React Native components, bound to break anytime
- Show an infinite loading indicator
- Style it just like the LoadConfigOverlay
    - Since it kinda represents the opposite, an "unload" then SDK is done

🔥🔥🔥

To kill componentWillMount, call destroyLocalTrack after mount.
Navigation to the blank page was synthetically forced and no
UI issues were noticed, possibly because destroyLocalTrack may
already be async so destruction may already have been occurring
after mount.

Unfortunately, as the Jitsi Meet development evolved the routing mechanism
became more complex and thre logic ended up spread across multiple parts of the
codebase, which made it hard to follow and extend.

This change aims to fix that by rewriting the routing logic and centralizing it
in (pretty much) a single place, with no implicit inter-dependencies.

In order to arrive there, however, some extra changes were needed, which were
not caught early enough and are thus part of this change:

- JitsiMeetJS initialization is now synchronous: there is nothing async about
  it, and the only async requirement (Temasys support) was lifted. See [0].
- WebRTC support can be detected early: building on top of the above, WebRTC
  support can now be detected immediately, so take advantage of this to simplify
  how we handle unsupported browsers. See [0].

The new router takes decissions based on the Redux state at the time of
invocation. A route can be represented by either a component or a URl reference,
with the latter taking precedence. On mobile, obviously, there is no concept of
URL reference so routing is based solely on components.

[0]: https://github.com/jitsi/lib-jitsi-meet/pull/779

* 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 basic indicator is extracted into a LoadingIndicator component,
which then NetworkActivityIndicator displays (or not) based on network
activity.

Implement a React Component which displays children as an overlay of
local video. The WelcomePage implemented such a component inside of it
among other WelcomePage-specific logic so I split
LocalVideoTrackUnderlay out of it. The new Component is used on the
BlankPage which may be displayed in the future not only when the
WelcomePage is disabled but also when there are long running network
requests, for example.

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

I'm not saying that the two commits in question were wrong or worse than
what I'm offering. Anyway, I think what I'm offering brings:

* Compliance with expectations i.e. the middleware doesn't compute the
next state from the current state, the reducer does;

* Clarity and/or simplicity i.e. there's no global variable (reqIndex),
there's no need for the term "index" (a.k.a "reqIndex") in the redux
store.

* By renaming net-interceptor to network-activity feels like it's
preparing the feature to implement a NetworkActivityIndicator React
Component which will take on more of the knowledge about the specifics
of what is the network activity redux state exactly, is it maintained by
interception or some other mechanism, and abstracts it in the feature
itself allowing outsiders to merely render a React Component.

It will only be shown when there are ongoing network requests.