Add support for pinned topics

[alya]

In zulip/zulip#19483, we plan to add support for organization-level pinned topics.

The mobile app should support this feature by sorting pinned topics to the top of their stream in Inbox and stream topics views, with a pin icon matching the web app.

We could also add a "Pin topic" option to the topic menu for users who have permission to do so. However, this is a rare action, and doesn't have to be supported in the legacy mobile app.

Web app / server issue:

• Add support for pinning topics within a stream zulip#19483