Support for writing and parsing FCP concerns with comment links

[anoadragon453]

The rust mscbot implementation would write concerns in the form:

• we need to spec exactly who is allowed to delete an attachment (MSC2278: Deleting attachments for expired and redacted messages matrix-spec-proposals#2278 (comment))

Whereas mscbot-python currently writes them as:

• we need to spec exactly who is allowed to delete an attachment

i.e, without the status comment.

So, we need to parse concerns with status comments attached. I imagine this will be quite simple with some regex.

It would also be nice to attach a link to the comment like the old bot used to do.

[anoadragon453]

Duplicate of #16