Combine code that finds mentions into one place and add hook points

Combine code that finds mentions into one place and add hook points

Combined the code that finds mentions of other profiles into one place.

common_find_mentions() finds mentions and calls hooks to allow
supplemental syntax for mentions (like OStatus).

common_linkify_mentions() links mentions.

common_linkify_mention() links a mention.

Notice::saveReplies() now uses common_find_mentions() instead of
trying to parse everything again.