Add Retrofit dependency.
Implement osTicket integration.

Users can now send feedback from within the app.
Display an error dialog if feedback can’t be sent.
Replace placeholder text.
Capitalise 'OK' in strings.
FeedbackDialogFragment implements the callback.
Use constants.
Clarifying comments, cleanup.
Clean up feedback dialog layout a bit.
Retain context in FeedbackDialogFragment.
Merge pull request #22 from learning-layers/feature/LL-821

Add feedback system.