Wrap localStorageService in arethusaLocalStorage

This way it will be easier at a later stage to switch out the
localStorage solution, if we ever have to.
Fix file name of pluginSetting directive
Start to implement userPreferences service
Let plugin settings communicate /w userPreferences
Add globalSettings.persistSettings
Read from userPreferences in configurator
Fix userPreferences.set
Minor refactor in arethusaLocalStorage
Coerce booleans from localStorage
Propagate globalSettings to userPreferences
Add globalSettings.name
Fix name of keyboardMapping globalSetting
Add and fix translations
Merge pull request #516 from latin-language-toolkit/more_local_storage

Saving user preferences