🚸(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>

src/backend/core/signals.py

@@ -4,12 +4,14 @@ Declare and configure the signals for the impress core application
 
 from functools import partial
 
+from django.core.cache import cache
 from django.db import transaction
 from django.db.models import signals
 from django.dispatch import receiver
 
-from . import models
-from .tasks.search import trigger_batch_document_indexer
+from core import models
+from core.tasks.search import trigger_batch_document_indexer
+from core.utils import get_users_sharing_documents_with_cache_key
 
 
 @receiver(signals.post_save, sender=models.Document)
@@ -26,8 +28,24 @@ def document_post_save(sender, instance, **kwargs):  # pylint: disable=unused-ar
 def document_access_post_save(sender, instance, created, **kwargs):  # pylint: disable=unused-argument
     """
     Asynchronous call to the document indexer at the end of the transaction.
+    Clear cache for the affected user.
     """
     if not created:
         transaction.on_commit(
             partial(trigger_batch_document_indexer, instance.document)
         )
+
+    # Invalidate cache for the user
+    if instance.user:
+        cache_key = get_users_sharing_documents_with_cache_key(instance.user)
+        cache.delete(cache_key)
+
+
+@receiver(signals.post_delete, sender=models.DocumentAccess)
+def document_access_post_delete(sender, instance, **kwargs):  # pylint: disable=unused-argument
+    """
+    Clear cache for the affected user when document access is deleted.
+    """
+    if instance.user:
+        cache_key = get_users_sharing_documents_with_cache_key(instance.user)
+        cache.delete(cache_key)