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>

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

[BD-24] [TNL-7661] [BB-3172] LTI Improvements - Use declarative grading model on XBlock launch (#116)

* create default LineItem, WIP grade save

* add score to django admin

* WIP: find user and save grade to xblock

* boolean pragramatic grade interaction flag and optional params in enable_ags method

* Submit grades using grade signals

* lineitem urls should be optional

* lineitem is now readonly in declarative method

* test grade_submit called properly

* quality issue

* raise LTIError

* moved listener to signal.py, refactored models.py, added due and start date, updated tests.

* use load_block_as_anonymous_user and remove load_block, refactor tests

* refactor test to fix quality issue

* make lineitems_url required

* refactor tests, accept_grades_past_due on check

* test accept_grades_past_due

* add comma to last items

* refactor get_lti_ags_lineitems_url

* make sure crum returns user and not None

* nitpicks & use maximum score when given score is larger than maximum

* fix docstring of load_block_as_anonymous_user

* 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

This change moves the LTI configuration retrieval to
a Django model that will LTI configuration.
The current model proxies back the to blocks to keep
backwards compatibility.

See reasoning for change at:
https://github.com/edx/xblock-lti-consumer/blob/master/docs/decisions/0001-lti-extensions-plugin.rst