TRACKING: Improve algorithm for 'next unread direct message' (p)

[neiljp]

This feature works minimally, but needs fixing/extending similar to #1332 (for topics).

This needs a reference to any discussions, and further clarification, but this needs to:

• [ ] cycle through in the correct order, as per the web app
• [ ] cycle through group PMs/DMs too
• [ ] ensure entry into a new conversation goes to the first unread message, not simply the bottom of the view

Split-out issues/PRs for this to date: worth completing first

• [x] Migrate method to model (ce9decc of #1395, fixing #1335)
• [ ] Refactor tests and tidy (remainder of PR #1395)

This will likely be easier to test (manually) with explore mode enabled, though tests are expected.