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.

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

The files styles.js are used (pretty much) on React Native only and each
of them exports 1 name. Export it as default to cut down on source code.

Looks like Android gets confused as to what surface to blit when we hide or
show toolbars. Setting a border on the container, seems to force the entire
area to blit properly.

Other attempted approaches, with no success:
- zIndex of -100
- width and height of 0
- opacity of 0 and setting 'disabled' on touch containers

This patch applies the workaround in the welcome page and conference containers.

As an intermediate step on the path to merging jitsi-meet and
jitsi-meet-react, import the whole source code of jitsi-meet-react as it
stands at
2f23d98424
i.e. the lastest master at the time of this import. No modifications are
applied to the imported source code in order to preserve a complete
snapshot of it in the repository of jitsi-meet and, thus, facilitate
comparison later on. Consequently, the source code of jitsi-meet and/or
jitsi-meet-react may not work. For example, jitsi-meet's jshint may be
unable to parse jitsi-meet-react's source code.