Show warning dialog when the expected timer differs from the current timer

This MR refactors message sending to return a LiveData that the TextSendController can observe to react to the sending state. The main reason is to be able to reliably catch automatic changes to the auto-delete timer which might surprise the user.

Test instructions:

• Add a contact, open private conversation screen and send a message
• Start typing a second message and activate disappearing messages, finish typing the message and send
  • check that a warning message appears warning about the changed timer
• Start typing a third message, but don't send
• Let your contact disable disappearing messages and send a message
• Wait for the contact's message to arrive and then send your message
  • check that a warning message appears warning about the changed timer

Closes #1864 (closed)