I see it as the first step in simplifying the route navigate of the
JavaScript app by removing BlankWelcomePage from _getRouteToRender. From
a faraway point of view, the app is at the route at which it is not in a
conference. Historically, the route was known as the Welcome page. But
mobile complicated the route by saying that actually it may not want to
see the room name input and join button.

Additionally, I renamed BlankWelcomePage to BlankPage because I don't
think of it as a WelcomePage alternative but rather as a more generic
BlankPage which may be utilized elsewhere in the future.

I plan for the next steps to:
* Merge Entryway, _interceptComponent, and _getRouteToRender in one
React Component rendered by AbstractApp so that the whole logic is in
one file;
* Get rid of RouteRegistry and routes.

When do we need tracks?

- Welcome page (only the video track)
- Conference (depends if starting with audio / video muted is requested)

When do we need to destroy the tracks?

- When we are not in a conference and there is no welcome page

In order to accommodate all the above use cases, a new component is introduced:
BlankWelcomePage. Its purpose is to take the place of the welcome page when it
is disabled. When this component is mounted local tracks are destroyed.

Analogously, a video track is created when the (real) welcome page is created,
and all the desired tracks are created then the Conference component is created.
What are desired tracks? These are the tracks we'd like to use for the
conference that is about to happen. By default both audio and video are desired.
It's possible, however, the user requested to start the call with no
video/audio, in which case it's muted in base/media and a track is not created.

The first time the app starts (with the welcome page) it will request permission
for video only, since there is no need for audio in the welcome page. Later,
when a conference is joined permission for audio will be requested when an audio
track is to be created. The audio track is not destroyed when the conference
ends. Yours truly thinks this is not needed since it's a stopped track which is
not using system resources.