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

• Migrate method to model (ce9decc of Migrated get_next_unread_pm and tests to model. #1395, fixing Migrate get_next_unread_pm & tests to model #1335)
• Refactor tests and tidy (remainder of PR Migrated get_next_unread_pm and tests to model. #1395)

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