Add support for BigBlueButton (BBB) recordings

[olberger]

Please follow the guide below

• You will be asked some questions, please read them carefully and answer honestly
• Put an x into all the boxes [ ] relevant to your pull request (like that [x])
• Use Preview tab to see how your pull request will actually look like

---

Before submitting a pull request make sure you have:

• At least skimmed through adding new extractor tutorial and youtube-dl coding conventions sections
• Searched the bugtracker for similar pull requests
• Checked the code with flake8

In order to be accepted and merged into youtube-dl each piece of code must be in public domain or released under Unlicense. Check one of the following options:

• I am the original author of this code and I am willing to release it under Unlicense
• I am not the original author of this code but it is in public domain or released under Unlicense (provide reliable evidence)

What is the purpose of your pull request?

• Bug fix
• Improvement
• New extractor
• New feature

---

Description of your pull request and other information

BigBlueButton (aka BBB) is a virtual classrooms tool that allows recording the classes. Its recordings playback application doesn't provide a recording download feature, bu default. Hence the potential use for this contribution.

Note that BBB records multiple feeds : speaker audio (and webcam) and screen sharing, while slides aren't recorded in video (later playback of slide images + annotations).

This work is related to https://github.com/trahay/bbb-downloader that was made by my colleague François Trahay.

[olberger]

I'm done for now.

Expect the tests downloads to fail after a while when my test recording will be purged. Couldn't spot a public instance of BBB making recordings available for a long time

[lebdron]

It would be great to integrate slides-to-video conversion from here https://github.com/createwebinar/bbb-download