Channel: Render age restricted channels

[ChunkyProgrammer]

closes https://github.com/iv-org/invidious/issues/3513

This PR:

• gets thumbnail and channel name from the initial request
• gets videos, shorts and streams via autogenerated channel playlists

Test Url: /channel/UCbfnHqxXs_K3kvaH-WlNlig

[github-actions[bot]]

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