video-provider: UI/style updates

- No more border radius in video containers, fullscreen buttons or user info span

Reduced the spacing between cameras to 1px

Replaced the loading spinners with a black background and the user name at the center

- Changed the color of the talking indicator to be the primary color with 0.7 opacity

Make the user name in the video container have opacity 1

Fix: make the webcam containers be 4:3 even while loading