Previously, the client tweaked messages sent in a channel with
an empty topic box to use the topic "(no topic)".
This commit updates the behavior to stop applying that tweak.
Messages will now be sent with an empty string as the topic
as we now support the empty string as a valid topic name.
This commit simplifies a compose_validate test by
using FakeComposeBox.
It also exposes $send_message_form to the callers.
(The general idea here is that for any individual
test, we don't want the developer to have to
switch between thinking about low-level zjquery
idioms and higher-level abstractions.)
This commit also removes the use of mock_template,
since the template in question generates trivially
easy html to deal with (and we verify the actual
effects to the DOM).
Refactors `check_overflow_text` and `validate_message_length` functions.
This commit ensures that there won't be any change in the existing UI
of the compose box with the refactoring of the above functions.
