🚸(backend) sort user search results by proximity with the active user (#1802)

## Purpose
Allows a user to find more easily the other users they search, with the
following order of priority:
- users they already share documents with (more recent first)
- users that share the same full email domain
- ~~users that share the same partial email domain (last two parts)~~
- ~~other users~~

Edit: We need to ilter out other users in order to not reveal email
addresses from members of other organisations. It's still possible to
invite them by email.

Solves #1521

## Proposal
- [x] Add a new function in `core/utils.py`:
`users_sharing_documents_with()`
- [x] Use it as a key to sort the results of a basic user search
- [x] Filter user results to avoid reveal of users (and email addresses)
of other orgs or that have not been interacted with.
- [x] User research through "full" email address (contains the '@') is
left unaffected.

---------

Co-authored-by: Anthony LC <anthony.le-courric@mail.numerique.gouv.fr>

CHANGELOG.md

@@ -13,6 +13,7 @@ and this project adheres to
 ### Changed
 
 - ♿️(frontend) Focus main container after navigation #1854
+- 🚸(backend) sort user search results by proximity with the active user #1802 
 
 
 ### Fixed