Comments: Show @-mentions in line with reply

[obenland]

Fixes #1135.

@pfefferle Any reason the mention handling shouldn't be part of get_content()? Is there a use case for it that requires it to not contain Fediverse mentions?

Proposed changes:

• Prepends mentions to comment content without wrapping paragraph tags.
• Moves @-mention handling into the comment content getter to avoid grepping HTML tags to insert them in the right place.
• Updates unit tests to test for the new expected output.

Other information:

• Have you written new tests for your changes, if applicable?

Testing instructions:

• Publish a post.
• Have another comment reply to the post on the Fediverse.
• On your site, reply to the comment via Federation.
• View the reply in the Fediverse client and make sure the reply is inline with the mentions.

[jeherve]

I'll say that this tests well on my site, but I'll let @pfefferle weigh in for your question. :)

[pfefferle]

@obenland thanks a lot, this should work totally fine! I have simplified the code a bit more and changed the HTML to unify with the one from the mention class.