Update success/error/warning messages

Messages that provide confirmation of actions, uploads and downloads need clarification in the app. Missing messages include:
- Point added/edited/deleted
- Deletion from "Your Contributions" screen
- Advanced settings - confirm deletions of caches, edits, state
- Message mentioned in #46

Messages to update:
- Save (for Add, Edit) dialogue confirmation, or "in process" animation (loading spinner, etc)
- UI for #45

_Edited: Messages for offline maps should be replicated throughout app_
_Edit 2:_ I now see the `Upload failed` message for adding points; will look at implementing this style for other messages where appropriate