Favorites are tracked at cipher level rather than on a per-user basis

[jjlin]

Currently, whether a cipher is a "favorite" is a property of the cipher itself, but it should probably be tracked similarly to folders, with a separate "favorites" table that maps users to ciphers. Otherwise, for a shared cipher, when one user sets a cipher as a "favorite", it automatically becomes every other user's favorite too, which probably isn't the intended behavior.

Also, when changing the "favorite" property, the cipher update time probably shouldn't be updated (similar to #884).