Update uses old migration path

[garethbowen]

An app can specify a migration_path to migrate the app_settings between versions which is a URL to an endpoint which takes the current settings and returns the migrated settings.

Currently the migration uses the migration_path in the old ddoc rather than the possibly updated one in the new ddoc. This means if the migration_path...

• ... is added between the old and new versions it won't be executed
• ... is modified between the old and new versions the wrong endpoint will be run
• ... is deleted between the old and new versions the migration request will 404

Modify the migration code to use the new ddoc configuration rather than the old one.