Fix seq generators for enum tables in postgres

This attempts to insert a temporary entry into each enum table until it
succeeds. It re-synchronizes the postgres sequence generators with the max id
of the table.

Fixes #883 and #880

data/migrations/versions/5ad999136045_add_signature_storage.py

@@ -38,12 +38,12 @@ def upgrade(tables):
 
     op.bulk_insert(tables.imagestoragetransformation,
     [
-        {'id': 2, 'name':'aci'},
+        {'name':'aci'},
     ])
 
     op.bulk_insert(tables.imagestoragesignaturekind,
     [
-        {'id': 1, 'name':'gpg2'},
+        {'name':'gpg2'},
     ])