"Author" label when a user is deleted

### As a Joplin moderator or administrator I want to assign pages to a user so that when a user is deleted or no longer owns a page someone will still be listed as the author.

**Is your feature request related to a problem? Please describe.**
Currently when a user is deleted, the pages they had owned are now assigned to "Unknown" user.

**Describe the solution you'd like**
Tell me what pages belong to a user when i click "delete" and give me a way to reassign them to one or more users.

**Describe alternatives you've considered**
Maybe default the ownership from the deleted user to their direct moderator?

**Additional context**
Add any other context or screenshots about the feature request here.