Improve knowledge of the course and assignments dates

For a number of features, we'll need to know, for example, when a course starts and ends, when assignments are available, and the deadline for student submissions.

In more detail, some of these features could include:

- Filter by dates/term in the dashboard

Include a date-aware filter on the LMS dashboard to filter by relevant dates/terms.

- Add an "add" badge for late enrollments

Display a badge next to students who were added to the course after the start date.

- Sync auto-graded assignments after deadlines

Automatically sync grades just after the course or assignment deadline.

- Prevent submissions after deadlines

Avoid generating Canvas submissions after the submission deadline.

### Gathering course dates via LTI variables

There are non-standar LTI custom variables we can use to fetch course dates and term information

#### Canvas

We started fetching course dates and terms one in Canvas for new installs

- https://github.com/hypothesis/lms/pull/6971
- https://github.com/hypothesis/lms/pull/6972
- https://github.com/hypothesis/lms/pull/6969
- https://github.com/hypothesis/lms/pull/6941
- https://github.com/hypothesis/lms/pull/6940
- https://github.com/hypothesis/lms/pull/6939

For schools that have this variable we should have full coverage of dates for new courses.

### D2L

We have updated the install docs to reflect new variables:

https://hypothes-is.slack.com/archives/C2BLQDKHA/p1739542083490679

The main ones being:

```
course_starts: $CourseSection.timeFrame.begin
course_ends: $CourseSection.timeFrame.end
```