Allow user individual tags (and favorites)

[butonic]

The favorite flag is a user specific property for a file that cannot be mapped to extended attributes without leaking who has marked a file as a favorite.

It is a specific case of a tag, which is user individual as well. I see these types of tags

1. public that are maganed by everyone
2. private (or user individual) tags that are only visible to the user
3. system tags that are only visible to the system
4. group tags that are only visible to a group ...

This can be solved using different namespaces or scopes for tags

1. public = p:
2. private = u:: for user specific
3. system = s: for system
4. group = g::
5. app? = a:: for apps?

Obviously this only is secure when the u/s/g/a namespaces are not accessible by users in the filesystem. public tags can be mapped to extended attributes, eg. dublin core metadata.

[butonic]

namespaces for metadata was already mentioned in https://github.com/cs3org/cs3apis/pull/31#issuecomment-530260184

[labkode]

@butonic this issue makes a lot of sense for favourites and any other metadata that user can attach to a shared resource like comments, chats, tags, ...

I've asked Andreas from the EOS team who is/was dealing with such dilemma for extended attributes.

If the extended attributes have support for such visibility we don't need to create a layer on top