✨(backend) search users

We need to search users by their email.
For that we will use the trigram similarity algorithm
provided by PostgreSQL. To use it we have to
activate the pg_trgm extension in postgres db.
To query the email we will use the query param
`q`.
We have another query param `document_id`, it is
necessary to exclude the users that have already
access to the document.

src/backend/core/migrations/0002_create_pg_trgm_extension.py

@@ -0,0 +1,14 @@
+from django.db import migrations
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('core', '0001_initial'),
+    ]
+
+    operations = [
+        migrations.RunSQL(
+            "CREATE EXTENSION IF NOT EXISTS pg_trgm;",
+            reverse_sql="DROP EXTENSION IF EXISTS pg_trgm;",
+        ),
+    ]