Support more variants of calls to coqbot

[Zimmi48]

Some people want to vary the message to use when calling @coqbot, for instance by adding "please". When it's added at the end, it works, but when it's added between the call to @coqbot and the directive, it currently doesn't.

In passing, we could stop ignoring messages to @coqbot that we don't understand. We could reply with something like "I didn't understand what you're asking. Here's what I can do." But this would require to be smarter to detect when it's a message to @coqbot and when it isn't. A reasonable heuristics would be to only take notice when @coqbot is at the beginning of a line. And everytime @coqbot doesn't understand something, it could also post a comment here to report about it.

[Zimmi48]

It might also be worth reacting to comments edited for typos.

[Zimmi48]

It might also be worth reacting to comments edited for typos.

This was implemented, but only for "issue comments" (as opposed to "review comments").