Replace usernames with uids in GalleryContent

[SilviaZeta]

Problem

We store usernames (i.e., users' email addresses) in different places inside a collection's content (or GalleryContent) and this is not great from a data protection perspective. For example, we do this when storing the assignees and to link audit and annotations items with users.

Solution

Replace all usernames stored in the GalleryContent with uids.

Considered Alternatives

No