UserGlobalStoragesService reads the global storage configuration,
cherry-picking storages applicable to a user. Writing storages through
this service is forbidden, on punishment of throwing an exception.
Storage IDs may also be config hashes when retrieved from this service,
as it is unable to update the storages with real IDs.
As UserGlobalStoragesService and UserStoragesService share a bit of code
relating to users, that has been split into UserTrait. UserTrait also
allows for the user set to be overridden, rather than using the user
from IUserSession.
Config\ConfigAdapter has been reworked to use UserStoragesService and
UserGlobalStoragesService instead of
OC_Mount_Config::getAbsoluteMountPoints(), further reducing dependance
on that horrible static class.
