Fixes#10124.
Users in the waiting period category cannot subscribe other users to
a stream. When a user tries to mention another unsubscribed user, a
warning message appears with a subscribe button on it to subscribe
the other user.
This commit removes the subscribe button and changes the warning text
for users in the waiting period category.
Instead of displaying a fixed error message inside the yellow bar itself,
now the yellow bar disappears on error and a red compose_error is shown.
The error message is the one returned from the server.
This refactors and fixes unicode issues where entities don't display
properly due to being a special character that seems to be rendered
incorrectly in a non-deterministic way every time.
Shubham Padia
Cynthia Lin
Brock Whittaker
Sanskar Modi
Umair Khan
Tim Abbott
Steve Howell
Leo Franchi