K-9 Mail version 18.0

New:
- PNG avatars
- Setting to display ISO date/time in message list
- Fixed-width font for composing

Changed:
- Attachment summary moved to header

Fixed:
- "Delete (from notification)" setting did not retain state
- Find folder search hint text was cut off
- User not notified if offline when sending message
- CC/BCC fields were not expanded by default for Reply All
- Crash occurred when retrieving images
- Deleted messages could remain visible in message list
