FIX: Ignore duplicate user_ids when migrating old data

The old storage mechanism had no unique index on the user_id, so it is possible to have bad data. In this case, take the first value, and ignore the rest.

db/migrate/20190724055909_move_to_managed_authenticator.rb

@@ -15,6 +15,8 @@ class MoveToManagedAuthenticator < ActiveRecord::Migration[5.2]
       CURRENT_TIMESTAMP
     FROM plugin_store_rows
     WHERE plugin_name = 'oauth2_basic'
+    ON CONFLICT (provider_name, user_id)
+    DO NOTHING
     SQL
   end
 end