by fixing the cause rather than the symptom: this upgrades the code to use the new, recommended JSX transform mode of React 17+, which no longer requires you to import React manually just to write JSX.
This version is not supposed to properly work, this is a work in
progress.
Main changes:
* Completely removed the PTT logic (for simplicity, it could be
introduced later).
* Abstracted away the work with the media devices.
* Defined confined interfaces of the affected components so that they
only get the data that they need without importing Matris JS SDK or
LiveKit SDK, so that we can exchange their "backend" at any time.
* Started using JS/TS SDK from LiveKit as well as their React SDK to
define the state of the local media devices and local streams.
This is an Element project (in the vector-im repo) so the Copyright
should be for New Vector: it was incorrectly attributed to the
foundation for some files (and some files were missing headers).
