Improve parsing of @mentions to not include e-mail addresses

[toebu]

If your comment contains an e-mail address ([email protected]) it will add "@example" to the CC. This should not happen. It should either add the whole e-mail address as a CC, or nothing at all. It would be great to be able to switch between these two modes in the config (some Trac instances use e-mail addresses as users, others don't).
In general the parsing should be improved to require a space or the beginning of a line before the "@" (and maybe also at the end of the name).

[pfiaux]

We could extend this to only update the cc if the target of the mention is one of the trac users. Then it wouldn't include non users which could be many use cases:

• email [email protected]
• typo @ICAN'ttype
• quote "make use you use @toebu on github"