video-provider: UI improvements

Backport PRs #8374 and its follow-ups #8888 and #8758 into 2.2

Improve Panopto MATCH_URL regular expression

kudos @pedrobmarin

I did it to force the placeholder to fill the grid, but it turns out it botches Safari and I dont know how to fix it yet

- 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

Some weird merged regressed #9407 when merging 2.2 in 2.3, and backporting the VP from 2.3 to 2.2 reintroduced the issue, so I reimplemented it

Add aria-label to whiteboard poll result

Prevent negative values being used in presentation height / width

Obsolete in html5 and also was breaking the message sanitizer

Only allow for https uri in welcome and modOnly messages

Allow img tag in welcome and modOnly messages

Add a minimum socket validation to full-audio connection

Sanitized modOnlyMessage prior to using it

Pulled HTML5 locales on July 29, 2020

Sanitize data v2.2 - Add tag br in whitelist