Federated mentions are not parsed correctly

[Nutomic]

Mentions from federated posts/comments are currently parsed from markdown using scrape_text_for_mentions(). This doesnt work with mentions coming from other platforms such as NodeBB or Mitras, because their mention format doesnt match our MENTIONS_REGEX.

To fix this we need to get mentions from the tag field (we are already writing that for mentions in outgoing posts). scrape_text_for_mentions() should only be used for posts created via local api, never for federation.

Here are two comments which should generate mentions but dont:

• https://lemmy.ml/post/39840360/22589433
• https://lemmy.ml/post/39840360/22592427