Fix broken tests

Add serializer for `link` content type

This implements the endpoint that LTI tools use to return the deep linking configuration to the LMS, along with the `ltiResourceLink` type to allow testing with demo tool.

Signed-off-by: Giovanni Cimolin da Silva <giovannicimolin@gmail.com>

* 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

* BD-24 Implement LTI AGS Score Publish Service and Results Service

* Address PR comments and add more validation

* Address PR comments

* Add tests; Fix error with scoreMaximum; Fix quality issues; Adjust user_id results url slightly

* Add permissions tests and address other PR comments

* Fix quality test

* Address PR comments

* Add initial version of LineItem Implementation

Signed-off-by: Giovanni Cimolin da Silva <giovannicimolin@gmail.com>

* Adding tests

* Add LTI tests and fix quality isses

* Quality nit

* Add missing requirement, upgrade

* Squash migrations

* Fix merge conflicts

* Addressing review comments

* Add django url testing support

* Add tests to django views, implement compat layer

* Addressing review comments

* Improve tests and test descriptions

* Nit

This commit adds support for the LTI 1.3 Access token endpoint, as detailed in the IMS Security Framework.
The token is generated using the consumer's private key (stored in the XBlock) after validating the message sent by the LTI Tool using it's public key.

Signed-off-by: Giovanni Cimolin da Silva <giovannicimolin@gmail.com>

Features:
Implement LTI 1.3 launch support.
Add LTI 1.3 passthrough views as LMS plugins.