Feature: Add ability for dispatchers to add signatures to their messages

[teesloane]

Describe your changes

• Adds optional dispatcher signature feature for outgoing SMS messages
• Dispatchers can enable "Signature on Messages" in their user settings to auto-append (sent by: Name) to messages they send
• Only affects messages sent by dispatchers through the UI; automated campaign messages are unaffected

My question - do we want to be adding settings on the user like this? Or maybe create a table just for settings, and then assign them in a many-to-many relationship with users? etc

Tests

• New unit tests for maybe_append_signature/2 function
• Tests for signature_on_messages field in accounts context
• Manual: Enable signature in user settings, send a message, verify signature appears
• Manual: Verify campaign messages do not include signatures

Screenshots

Checklist before requesting a review

• I have performed a self-review of my code
• If it is a core feature, I have added tests.
• Are there other PRs or Issues that I should link to here?
• Will this be part of a product update? If yes, please write one phrase
  about this update in the description above.