Implement add-attachment use case

[octo]

This is for the ability to add attachments when composing new messages. It requires at least the following things to be implemented and/or considered:

• UI for adding attachments during composition
  • Custom dialog on <5.0
  • File Picker API on 5.0
• Need to handle outgoing message serialization cases
  • Drafts
  • Outbox
• Need to handle sending messages with attachments
  • Probably trivial since the existing MIME serialization should handle it
• Need to consider forwarding messages with attachments, which may or may not be realistic. (all depends on whether or not those attachments have been downloaded)

[octo]

Implemented.

Decided to use a custom-written file picker for all platforms. May consider the built-in one for 5.0 in the future.

There may be some performance/memory issues with larger attachments, and there is currently no data chunking or size detection/scaling/warning, so these concerns may have to be addressed in the future.