Improve empty message views in unsubscribed streams

[alya]

We currently show a custom message when a user views a stream they are not subscribed to that has no messages in it. This is an unusual situation, and we should make it work more similarly to other contexts:

• [x] Show the standard empty message feed placeholder. ("There are no messages here. Why not start the conversation?")
• [ ] Style the "Subscribe" button as it's styled in a non-empty stream.
• [ ] Show the "Subscribe" button when viewing an empty topic in an unsubscribed stream (currently missing).

Follow-ups:

• [ ] It would be best to also show the "You are not subscribed..." bookend, but as that part is more tricky, let's make it a follow-up.

CZO thread

[nimishmedatwal]

@zulipbot claim