In-app banners for messaging

[rnystrom]

Show some remotely-controlled (Firebase) banners, similar to the rating control, that we can show on demand.

• Content
  • title: String
  • message: String
  • imageURL: URL? (remotely load, fixed size to begin)
  • link: URL?
• X button to dismiss
  • Each banner should have a uniqueID: String that we store in UserDefaults somehow. Once you close it, never display that banner again.
• Tap optional link to open in browser

This would be useful for stuff like:

• Promote new features
• Apologize when Ryan ships a shitty build
• TestFlight signups
• ???

[BasThomas]

Do we still want this? New features now are somewhat sherlocked by the release notes, TestFlight signups are sherlocked by the apple URL thing and the option in Settings.

[Sherlouk]

Did someone say Sherlock?