Create PreferencesRepository, transfer all managing data on viewModel

[x] Create PreferencesRepository which load and store all the preferences info: UserInfo, ProfilePicture, TimeSlotDetails
[x] Move all data management inside the VM (both for UserProfile and TimeSlotDetails)
[x] Edit will call vm.saveData() that will both update preferences and LiveData object
[x] Show will receive an already updated VM (which means that the first time the main activity or the VM will call the repo.loadData()

A-short-name / G01-Timebanking