Store ID of user who creates collection item on the collection item

When a collection item is created, e.g., by a creator adding a work or bookmark to a collection or a maintainer inviting a work, we should store the ID of the user who prompted the item’s creation on the collection item for our own edification.

It should stay as long as the collection item exists (i.e., don’t nil it out if the work creator removes themselves from the work or the collection moderator removes themselves from the collection).

We can call it collector_id or item_creator_id or some such.

It shouldn’t be displayed anywhere for now. Database admins will need to test.