otherwise this blows up, trying to migrate a record with no location
(the modulestore lookup throws).

Presumably this isn't (wasn't?) an issue in prod already, right?
But I did hit this in my devstack, having previously created an
LtiConfiguration record via Django Admin (without a location).

* Move LTI 1.3 Key management to model

This:
- Removes the need to load the modulestore on every public keyset endpoint call.
- Simplifies the block structure and parent method overrides.
- Removes private key, client id and related parameters from XBlock fields

It also includes a migration from the data stored in the block to the model.

* Cleanup unused test helpers

* Version bump

* Addressing review comments