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/4a0c94399f38_add_new_notification_kinds.py

@@ -16,9 +16,9 @@ import sqlalchemy as sa
 def upgrade(tables):
     op.bulk_insert(tables.externalnotificationmethod,
     [
-        {'id':4, 'name':'flowdock'},
-        {'id':5, 'name':'hipchat'},
-        {'id':6, 'name':'slack'},
+        {'name':'flowdock'},
+        {'name':'hipchat'},
+        {'name':'slack'},
     ])
 
 def downgrade(tables):