initial favorites support

[butonic]

good enough for testing

• only works for the owncloud storage driver
  • eos and actually any storage drive would have to take into account if a property is user specific or not...
    • user individual metadata sucks
  • storing them in extended file attributes would leak favorites (and tags) but more flawed: users can make any of their owned files a favorite for another user by writing the corresponding xattr ... we need to check if we can use a xattr namespace (TRUSTED?) that requires a different privilege... see CAP_SYS_ADMIN and how to set it
    • CAP_SYS_ADMIN way to powerful a capability. friends don't let friends run reva as root ...
• we can learn from this and reimplement favorites on top of a proper tagging service
  • raised discussion in Allow user individual tags (and favorites) cs3apis#43

[labkode]

@butonic I merge now but let's follow up on the discussion on CS3, I'll involve Andreas from EOS as he is dealing with the same challenge :)