This causes index offset issues with any rich elements in the message.
We could try to fix the offsets, or alternatively replace the collapsed
newlines with spaces (to maintain the original offsets), but I don't
think it's worth the effort since I'm not sure that it's a good idea to
collapse newlines in the first place.
to determine valid characters to form a boundary before an `@` mention
Also fixed an issue with mentions looking like they're part of URLs, by
first processing mentions separately.
Fixes#1083
Directives are rendered as templates and their bodies are MessageText instances.
We thereby achieve the necessary nesting of directives (and other rich
elements inside directives) by letting each directive
body render itself similarly to how the whole message body is rendered.