DB configuration gets changed after loading an XBlock in Studio

[Agrendalath]

Steps to reproduce

1. Go to Studio and create a new LTI Consumer XBlock.
2. Set the Configuration Type to CONFIG_ON_DB.
3. Go to the DB configuration of this XBlock and set the configuration to CONFIG_EXTERNAL.
4. Load the XBlock in Studio.

What happens

The CONFIG_EXTERNAL in the LtiConfiguration DB entry is changed to CONFIG_ON_DB.

What should happen?

The LtiConfiguration DB entry should remain intact.

Originally posted by @Agrendalath in #390 (review)