Slight refactoring of Settings tab and model

This change makes interaction with settings model unified in all
the places it is used. It also introduces mixin for models that will
allow to efficiently cache its data and to minify amount of GET
requests. This change will simplify implementation of role/settings
dependecies.

Change-Id: I67d0790d08152884f822b62da5ec5d58e932dc50
Merge "Slight refactoring of Settings tab and model"