[PeerTube] Do not accept non-URLs

[petlyh]

• I carefully read the contribution guidelines and agree to them.
• I have tested the API against NewPipe.
• I agree to create a pull request for NewPipe as soon as possible to make it compatible with the changed API.
  No NewPipe changes are needed.

This PR makes the PeerTube onAcceptUrl methods return false if the input is not a valid URL.
This is done by passing the input to the URL constructor and returning false if a MalformedURLException gets caught.

The PeerTube onAcceptUrl methods currently only checks whether the input contains path parts like /videos/ or /a/.
This causes problems when they get called when NewPipes SearchFragment.search checks whether the users search query is an accepted URL, since non-URL search queries can contain those parts. TeamNewPipe/NewPipe#10695 is caused by the search query orchestr/a/ being interpreted as a PeerTube Account URL because it contains /a/.

Fixes TeamNewPipe/NewPipe#10695

[TobiGr]

Thank you! Can you also add test cases to ensure we won't introduce this unwanted behaviour again? (e.g. here)

[TobiGr]

Thanks. I added the tests