[hhu] Add new extractor

[YtvwlD]

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
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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

HHU Mediathek contains videos created by the Heinrich-Heine-Universität Düsseldorf (mostly recordings of lectures).
This extractor allows the download of single, publicly visible videos.

[dstftw]

Read coding conventions.

[YtvwlD]

I read the coding conventions before but I didn't notice the rule regarding closing braces/brackets/parentheses. (I'm used to the guidelines in PEP8, sorry about that.)

[dstftw]

It does not say anything about brackets and braces. Also mostly it's nothing to do with closing parentheses which are perfectly valid PEP8.
As for extractor: jwplayer is already covered by generic extractor that should be extended to support such cases.

[YtvwlD]

The generic extractor doesn't work here because _find_jwplayer_data doesn't recognize the player config because the used regex fails at the encodeURI call.

I modified my PR to at least use _parse_jwplayer_data.
Also, I had to touch the regex in _parse_jwplayer_formats to also accept format descriptions that are not at the beginning of the label text. I really hope that this won't break existing stuff - I tested a few extractors using these functions, but most of them seem have broken already before my change.

[dstftw]

As already said jwplayer extraction code should be extended to support sanitizing function calls out of options JSON. HHU extractor must be removed. Extraction should be processed by generic extractor.

[YtvwlD]

For finding the options there is already #21132 which works in this case (but isn't merged yet).

So, I'm guessing I should close this PR and open two new ones - one for dada9f6 and the other one for parsing this JS?