[opencast] Add new extractor

[C0D3D3V]

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• 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

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What is the purpose of your pull request?

• Bug fix
• Improvement
• New extractor
• New feature

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Description of your pull request and other information

Opencast is a platform where universities, especially in Germany and Austria, can publicly upload lectures. There is also the possibility to register as a student with pseudo accounts which are created for lectures. However, the login is not implemented (but you can bypass it with a cookie).
I picked out a few of the public Opencast instances (with the help of Google). If you look at the domains, most of them have a login mask, but this is not important for watching the videos, you can find the videos under the path /engage/ui/index.html?s=1&p=1.
There are two big video players on Opencast 'paella' and the old player 'theodul' . I currently only have the links for paella as valid. Theodul is no longer maintained anyway and is rarely used by any university.