Rewrite plain-text @nick as an IRC highlight

[rollecode]

On Beeper, manually typing @alice hello (not using the client's mention autocomplete, so no pill, no m.mentions) currently relays to IRC as the literal string @alice hello instead of the usual IRC highlight convention alice: hello.

This only rewrites the message when the word right after @ matches a nick actually on the channel right now (is_on_channel), so it never invents a mapping for a name that isn't there.

Related but distinct from two earlier threads worth reconsidering here:

• Detect fluffychat fallback mentions and remove leading @ #146 wanted to strip a leading @ from FluffyChat's specific fallback-mention text and got shelved in favor of doing mentions properly once formatted_body/pills were supported (Use formatted_body for message text  #102, since closed). This isn't that: there's no pill, no fallback-mention markup at all, just plain manually-typed text, so the "handle it properly via Use formatted_body for message text  #102" answer doesn't apply here.
• Real mentions for Matrix users based on their Displayname? #195 raised concerns about mapping Matrix users to IRC nicks by displayname being ambiguous. This doesn't map anything: it only fires when the exact word is already a live nick on the channel, so there's nothing to disambiguate.

Given that, happy to discuss if this should still be opt-in via a room setting, but wanted to raise it since the earlier objections don't seem to carry over directly.

[rollecode]

Generalized this: the first commit only rewrote a mention opening the message. Second commit strips @ anywhere in the text (still gated on is_on_channel), since what's up @nick was still showing the literal @ mid-message.