* Change the implementation of GetAll() in `preference_store.go` to return an
object from `model` and a *model.AppError.
Change the Interface in `store.go` to accomodate for the change
Change the test that called the GetAll() function
* Rename the result variable to preferences so it makes more sense.
Use assertions to keep the test consistent
* Generate the correct mocks after the code changes
* Remove redundant conditions
* Address govet errors
* Resolve conflicts with master's new changes
* Fix Save() function according to the new changes in master (Got
overwritten with the previous commit)
* Change the assertions to have the same format as commit 2d97f01
* response format changes
* Generated mocks
Fixed all references of Preferences.Save
* Remove old code from store.go (incorrect merge)
* Review change - Add validations on count and error
* Review change - return from root level
* Review change - return 0 as nil value for int
* fix initialisation of err in preference_store
* modify Get method in preference_store
* modify interface to match changes on preference store Get method
* modify variable initialization
* fix behavior on GetPreferenceByCategoryAndNameForUser
