Add substitution variables to config

The required launch parameters provide minimally functional information about a given session. We want to reliably collect some additional information about courses and assignments, for both feature work and reporting.

The standard way to accomplish this is through substitution variables, which allows us to ask for exactly what we want to receive--contingent on the LMS supporting it.

## Constraints

We want to collect the following for the following reasons

- ResourceLink.title
- human readable reporting on assignments
- to show the title of the assignment in app
- ResourceLink.description
- human-readable reporting
- to show assignment instructions in app
- ResourceLink.available.startDateTime
- reporting: identify "planned" assignments which indicates "embedded" use, a KPI
- to notify students when their assignment is available
- ResourceLink.submission.endDateTime
- report on completion rates
- automatically submit for students
- ResourceLink.id.history
- facilitate more reliable course copy
- Context.id.history:
- facilitate more reliable course copy

## Notes

- Visit annotations on [Canvas API docs](https://hyp.is/go?url=https%3A%2F%2Fwww.imsglobal.org%2Fspec%2Flti%2Fv1p3&group=i3Kv4jaP) and [IMS](https://via.hypothes.is/https://canvas.instructure.com/doc/api/file.tools_variable_substitutions.html) for additional notes