Add support for prepared message templates

[RalfJung]

There are some messages I end up typing over and over since each contributor needs to learn about our tooling. For example:

@rustbot author

Please write `@rustbot ready` when the PR is ready for another round of review.

Or

This looks good, thanks! Please squash the PR into a single commit.
Use `git rebase --interactive --keep-base ...` to do that so that there's no unnecessary diff.

@rustbot author

It would be nice if I could configure some repo-specific help messages so that I don't have to type the same thing over and over again. (I won't always have the time to be as patient as above on explaining the squashing procedure.)

So for instance:

@rustbot author-help
@rustbot author-squash

could act like @rustbot author but also print a message configured in triagebot.toml. (I am not sure if it is a good idea to tie this to @rustbot author, but both of the most prominent examples in my interaction happen to be of that sort.)

[apiraino]

Would the GitHub Saved Replies solve your need? (I use them, too)

[RalfJung]

I didn't know about that feature. Will give it a try, thanks!

[Urgau]

I'm curious @RalfJung, did you have a chance try to out GitHub Saved Replies?

Does it still make sense to support prepared message in triagebot?

[RalfJung]

Yeah I've tried those now and they work great. :)

That said, it could still be useful for triagebot to support something like this so that messages can be shared across maintainers.