add default parameter for AllConfig->get*Value()
Also add default to the \OCP\IConfig interface
cache the home folder of a User
fix fallback overwriting result of getHome
fix the config option to remove the ability for users to set their displayname
Replace static usage of OC_Config and OC_Preferences with the injected \OC\ConfigAll
Pass config object in testGetHomeNotSupported
Merge pull request #6612 from owncloud/user-no-change-displayname-stable

Backport: Re-add the config options to remove the ability for users to change their displayname