[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