UI tweaks

[oxzi]

This pull request introduces some changes to the BigBlueButton web UI to be displayed within the stream. As those cover different scopes, each is in its own commit.

In a nutshell, the

• first commit hides some interactive UI elements one simply cannot click in a video.
• second commit introduces two flags BBB_HIDE_MEETING_TITLE and BBB_HIDE_WHO_TALKS which allow hiding the relevant bar at the top.
• third commit allows overriding the background color from BBB's blue to whatever color one prefers.

More information is available in the commit messages.

[mtsonline]

@oxzi I am sorry, but could you please resubmit all your prs from this one on and make sure they have no conflixts? I had to rebase the repo due to conflicts, which could not be solved easily, so I choose to go this way to keep everything clean (beside the pr history on github) ;-) Thank you.