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.

The name better suits its purpose, since it can be called at any time.

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

The end goal of this patch was to avoid opening the camera when there is no
welcome page.

In order to achieve this, the logic for creating the local tracks was
refactored:

Before this patch local tracks were created when lib-jitsi-meet was initialized,
and destroyed when it was deinitialized. As a side note, this meant that when a
conference in a non-default domain was joined, local tracks were destroyed and
recreated in quick succession.

Now, local trans are created and destroyed based on what the next route will be,
and this happens when the target room has been decided. This allows us to create
local tracks the moment we need to render any route, and destroy them when there
is no route to be rendered. As an interesting byproduct, this refactor also
avoids the destruction + recreation of local tracks when a conference in a
non-default domain was left.