folder_branch_ops: add methods for self-favorites management
folder_branch_ops: add methods for self-favorites management

Since FBO has the head/handle object, it makes sense for it to manage
its own favorites status.

Issue: KBFS-848

keybase/client,keybase/kbfs,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/kbfs,keybase/client,keybase/client