model/boxes: Allow users to post stream messages without a topic.

[kaustubh-nair]

This follows the convention used by other Zulip clients to
automatically post a message without a topic, or a topic with only
whitespaces, to a topic named '(no topic)'.

Tests amended.

Fixes #754.

[neiljp]

@kaustubh-nair This looks good functionally 👍 I left two minor points inline; otherwise my only thought is whether we should add a test for the UI, since we dropped them in the case where the functionality was elsewhere?

[kaustubh-nair]

@neiljp Thanks for the review! This is ready

[neiljp]

@kaustubh-nair Your first commit claims to have added a test, but there is only a test in the second commit. Did you mean to have a simple test in that first commit and then extend it in the second?

[kaustubh-nair]

@neiljp That's what I had planned before, but the logic in the first commit is too simple to require a test, so I thought I'd just add one for topic matching in the second one. It would just involve checking if the model methods are called with mocked parameters.

[neiljp]

@kaustubh-nair Thanks - I just wanted clarification on the test before merging, though I also combined some test lines in the mocking before merging manually. In any case, another good merge! 🎉