🚸(backend) use unaccented full name for user search

We have the user full name through OIDC in the database, but the search only
used the email field.
This change allows to search for a user by their first and/or
last name (fix #929).
Given that user names are more likely than emails to include diacritics, it
unaccents both the query and the database entry for search (fix #1091).
It also unaccents for email so that internationalized domain names are
managed whether or not the accent is included in the search.
An unaccented gin index is added on users full_name an email fields.
Using a manual migration because a wrapper around unaccent is necessary
to make it IMMUTABLE (cf.
https://stackoverflow.com/questions/9063402/ )

src/backend/core/api/viewsets.py

@@ -1,4 +1,5 @@
 """API endpoints"""
+
 # pylint: disable=too-many-lines
 
 import base64
@@ -18,7 +19,7 @@ from django.core.validators import URLValidator
 from django.db import connection, transaction
 from django.db import models as db
 from django.db.models.expressions import RawSQL
-from django.db.models.functions import Left, Length
+from django.db.models.functions import Greatest, Left, Length
 from django.http import Http404, StreamingHttpResponse
 from django.urls import reverse
 from django.utils import timezone
@@ -37,6 +38,7 @@ from rest_framework import response as drf_response
 from rest_framework.permissions import AllowAny
 
 from core import authentication, choices, enums, models
+from core.api.filters import remove_accents
 from core.services.ai_services import AIService
 from core.services.collaboration_services import CollaborationService
 from core.services.converter_services import (
@@ -188,13 +190,15 @@ class UserViewSet(
             queryset = queryset.exclude(documentaccess__document_id=document_id)
 
         filter_data = filterset.form.cleaned_data
-        query = filter_data["q"]
+        query = remove_accents(filter_data["q"])
 
         # For emails, match emails by Levenstein distance to prevent typing errors
         if "@" in query:
             return (
                 queryset.annotate(
-                    distance=RawSQL("levenshtein(email::text, %s::text)", (query,))
+                    distance=RawSQL(
+                        "levenshtein(unaccent(email::text), %s::text)", (query,)
+                    )
                 )
                 .filter(distance__lte=3)
                 .order_by("distance", "email")[: settings.API_USERS_LIST_LIMIT]
@@ -203,11 +207,15 @@ class UserViewSet(
         # Use trigram similarity for non-email-like queries
         # For performance reasons we filter first by similarity, which relies on an
         # index, then only calculate precise similarity scores for sorting purposes
+
         return (
-            queryset.filter(email__trigram_word_similar=query)
-            .annotate(similarity=TrigramSimilarity("email", query))
+            queryset.annotate(
+                sim_email=TrigramSimilarity("email", query),
+                sim_name=TrigramSimilarity("full_name", query),
+            )
+            .annotate(similarity=Greatest("sim_email", "sim_name"))
             .filter(similarity__gt=0.2)
-            .order_by("-similarity", "email")[: settings.API_USERS_LIST_LIMIT]
+            .order_by("-similarity")[: settings.API_USERS_LIST_LIMIT]
         )
 
     @drf.decorators.action(