### Objective
* Prevent data loss when the user inadvertently hits back or wants to
leave the profile edition with unsaved changes.
### Description
* To limit the number of changes to the existing codebase, I merely
re-used the same method used by `save()` in the ViewModel to decide
whether to make a network request or simply return the profile as-is.
* ~A bit of code juggling around in the ViewModel and I was able to use
the logic for all the encoding of each profile field (Which is what the
ViewModel caches in memory).~ Thanks @Lakoja for improving this in the
VM.
* A couple of internal data classes used as helpers to move all the
fields around (now that they are no longer used in one single place)
were introduced.
### Potential Optimizations
* ~The profile encoding is done twice (once for checking, and then again
if the user has to actually save it). I'd say this is a negligible price
to pay, since the alternative would be to create a different set of
comparisons and/or keeping another profile in memory for the purpose of
comparison.~
### Visual Improvement
* I believe the Dialog is difficult to see, but it's being displayed
with Tusky's theme. Perhaps there's a better style to apply in this
case? (or maybe the edit profile activity shouldn't have the same
background color as dialogs?!)
### Issue
* #3486
10a6b1616a