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.
