Trans: Use SQL functions to migrate data

That is way faster than doing that in Python

weblate/trans/migrations/0091_json_key.py

@@ -1,6 +1,8 @@
 # Generated by Django 3.0.7 on 2020-07-23 08:41
 
 from django.db import migrations
+from django.db.models import F
+from django.db.models.functions import Substr
 
 
 def report_progress(previous, current):
@@ -20,18 +22,10 @@ def migrate_componentlist(apps, schema_editor):
             "i18next",
             "webextension",
             "json-nested",
-        )
+        ),
+        context__startswith=".",
     )
-    count = units.count()
-    if not count:
-        return
-    progress = -1
-    for pos, unit in enumerate(units.iterator()):
-        if unit.context.startswith("."):
-            unit.context = unit.context[1:]
-            unit.save(update_fields=["context"])
-        progress = report_progress(progress, 100 * pos // count)
-    report_progress(progress, 100)
+    units.update(context=Substr(F("context"), 2))
 
 
 class Migration(migrations.Migration):