[BD-2][TNL-7315] BB-2612: Add ADR for LTI 1.3 variable definition (#89)

* Add ADR for LTI 1.3 variable definition

* Update docs/decisions/0002-lti-1p3-variables.rst

Co-authored-by: Ned Batchelder <ned@nedbatchelder.com>

* Update docs/decisions/0002-lti-1p3-variables.rst

Co-authored-by: Ned Batchelder <ned@nedbatchelder.com>

* Update docs/decisions/0002-lti-1p3-variables.rst

Co-authored-by: Ned Batchelder <ned@nedbatchelder.com>

* Update docs/decisions/0002-lti-1p3-variables.rst

Co-authored-by: Ned Batchelder <ned@nedbatchelder.com>

* Update docs/decisions/0002-lti-1p3-variables.rst

Co-authored-by: Ned Batchelder <ned@nedbatchelder.com>

* Update docs/decisions/0002-lti-1p3-variables.rst

Co-authored-by: Ned Batchelder <ned@nedbatchelder.com>

* Update docs/decisions/0002-lti-1p3-variables.rst

Co-authored-by: Ned Batchelder <ned@nedbatchelder.com>

* nit

* Remove student role from instructor/staff launches

Co-authored-by: Ned Batchelder <ned@nedbatchelder.com>