[SwitchTube] Add new extractor

[belamenso]

Before submitting a pull request make sure you have:

• [x] Searched the bugtracker for similar pull requests
• [x] Read adding new extractor tutorial
• [ ] Read youtube-dl coding conventions and adjusted the code to meet them Mostly, some lines are often longer than 80 columns, and no data point is extracted from more than a single source.
• [x] Covered the code with tests (note that PRs without tests will be REJECTED)
• [x] 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:

• [x] 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
• [x] New extractor
• [ ] New feature

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Description of your pull request and other information

This pull request adds tube.switch.ch, probably the largest Swiss website for universities to share lecture videos.

[belamenso]

I updated the code either exactly as suggested or similarly.

[belamenso]

I believe I was missing from __future__ import unicode_literals. I added it.

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Can someone please approve the CI tests? @dirkf

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I implemented the suggestions.

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Can somebody look into this?

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SwitchTube has apparently changes the playlist website format, making a large part of this extractor useless.

PLEASE DO NOT MERGE.

I will try to fix it in a week.

[belamenso]

I've split the previous SwitchTubeProfileOrChannelIE into SwitchTubeProfileIE (which was not changed on the website) and SwitchTubeChannelIE (which changed). It should work again now.