Enable moderators to add templates for mod messages

[cellio]

This has come up in chat a few times. We have some built-in templates for moderator messages, but no way for moderators to add new ones on their communities. This means moderators keep local copies or stash templates in chat or copy/paste from previous mod messages -- all of which work, but are clumsy.

After the mod-message changes from #767 are finished (let's not make finishing that any harder than it already is), can we add some way to add more templates? Ideally moderators could add locally and admins could add globally. I suspect the current templates are baked in, so we'd need to rework how templates are stored, which is why I don't want to do it before those other changes.