Allow users to share a document with other users at the same or lower role

So currently only admins can invite (or even see permissions). I think it would be better to allow users to invite other users at the role they have or lower. (Internally, it would be in fact allow users to invite other users and give them permissions they have or a subset of them.)

Also, everyone should be able to see all permissions on the document, if they have access to the document. I think this is important for accountability and transparency.