Optimize how we fetch the latest (current) revision

The way were were doing it was implicitly sorting the query items and it
was causing millions of items to be sorted (even though the result should
only have one) making it slow.

By switching away from `get()` and `first()` we are telling django to
not try to sort.

etebase_server/django/models.py

@@ -96,7 +96,7 @@ class CollectionItem(models.Model):
 
     @cached_property
     def content(self) -> "CollectionItemRevision":
-        return self.revisions.get(current=True)
+        return self.revisions.filter(current=True)[0]
 
     @property
     def etag(self) -> str:

etebase_server/fastapi/routers/collection.py

@@ -396,7 +396,7 @@ def item_create(item_model: CollectionItemIn, collection: models.Collection, val
         if not created:
             # We don't have to use select_for_update here because the unique constraint on current guards against
             # the race condition. But it's a good idea because it'll lock and wait rather than fail.
-            current_revision = instance.revisions.filter(current=True).select_for_update().first()
+            current_revision = instance.revisions.filter(current=True).select_for_update()[0]
             assert current_revision is not None
             current_revision.current = None
             current_revision.save()