Hopefully this does a better job of spurring people to action, and also
suggests a self-service fix if they don't (i.e. contacting the person that
invited them).
For almost all of these, the verbose support offer clause is almost the same
as the non-verbose version. The distinction carries extra carrying cost when
coming up with new copy, testing new email designs, etc.
Make sure 's, &s, and other characters are not HTML-escaped in subject
lines and plain-text emails.
Hack so that this isn't blocking the release of Zulip 1.6. A more robust way
to do this would be to have two different template Engines, one that renders
HTML, and one that doesn't.
Fixes#5088.