Add a call toolbar and end-of-call warning

What does this MR do?

This removes most default buttons on the Jitsi call and adds our own, in a custom bar. It also adds an end-of-call warning, 2 minutes before the meeting ends. We don't yet update the background of the microphone/video buttons when they're toggled (that's for later), but the buttons are functional currently.

Closes #86 (closed)

Closes #56 (closed)

Screenshots

Show what changed to the program (if it affects ui)

before

after

Jan the Penguin:

The end of meeting timer message: (it pops up on the right of the toolbar 2 min. before the slot ends)

Does this MR meet the acceptance criteria?

• I have added tests to validate the new functionality.
• I have updated the documentation accordingly.