Migrations: Use per role settings in PostgreSQL instead of per databa…

…se ones

In some cases people have more schemas in the database and touching the
database settings is not allowed.

Also remove duplicate tweaking of the parameter, the old value setting
is not necessary.

Fixes #3940

weblate/memory/migrations/0007_use_trigram.py

@@ -6,18 +6,6 @@
 def create_index(apps, schema_editor):
     if schema_editor.connection.vendor != "postgresql":
         return
-    # This ensures that extensions are loaded into the session. Without that
-    # the next ALTER database fails unless we're running as superuser (which
-    # is allowed to set non existing parameters, so missing extension doesn't
-    # matter)
-    # See https://www.postgresql.org/message-id/6376.1533675236%40sss.pgh.pa.us
-    schema_editor.execute("SELECT show_limit()")
-
-    schema_editor.execute(
-        "ALTER DATABASE {} SET pg_trgm.similarity_threshold = 0.7".format(
-            schema_editor.connection.settings_dict["NAME"]
-        )
-    )
     schema_editor.execute("DROP INDEX memory_source_fulltext")
     schema_editor.execute(
         "CREATE INDEX memory_source_trgm ON memory_memory USING GIN (source gin_trgm_ops)"

weblate/memory/migrations/0008_adjust_similarity.py

@@ -14,8 +14,8 @@ def update_index(apps, schema_editor):
     schema_editor.execute("SELECT show_limit()")
 
     schema_editor.execute(
-        "ALTER DATABASE {} SET pg_trgm.similarity_threshold = 0.5".format(
-            schema_editor.connection.settings_dict["NAME"]
+        "ALTER ROLE {} SET pg_trgm.similarity_threshold = 0.5".format(
+            schema_editor.connection.settings_dict["USER"]
         )
     )