Draft: Agents for access groups

This includes a major change in the access rights of uploads. User ids can be added to the coauthor_groups/reviewer_groups fields or their writer_groups/viewer_groups counterparts and are compared to a list of the group ids the sender is owner or member of and their user id. The union of user and group is called agent in the code (cf. Friend of a Friend ontology) but the groups fields keep their name for backwards compatibility.

Upload tests for group/agent are extended and existing tests merged.

The coauthors/reviewers fields and their writer/viewer counterpart are marked as deprecated.