Add feature flag for dashboard segments filter

[acelaya]

This PR adds a feature flag to control when to show the segments filter in the dashboard assignment view.

The feature flag will be taken into consideration only for non-staff users. For staff users, the BE will always expose this option as enabled.

Additionally, for auto-grading assignments, the FE does not take this option into consideration, and instead, it always shows the filter, as it is important in that case, and the first use case for which we introduced the segments filter.

Testing steps

1. Check out this branch
2. For staff:
   1. Go to the courses section in the LMS admin http://localhost:8001/admin/courses
   2. Click search, open any course from the result, and click on "Open instructor dashboard"
   3. Once there, if you select any assignment, you should see the segments filter, with a total of 4 dropdowns.
3. For auto-grading assignments:
   1. Go to https://hypothesis.instructure.com/courses/319/assignments/7296
   2. Click on the user icon in the sidebar, then in "Open dashboard".
   3. You should see the segments filter, with a total of 4 dropdowns.
4. For assignments where the feature is explicitly enabled:
   1. Go to https://hypothesis.instructure.com/courses/125/assignments/873 and log in as eng+canvasteacher@hypothes.is
   2. Click on the user icon in the sidebar, then in "Open dashboard".
   3. You should only see three filter dropdowns.
   4. On a new tab, go to http://localhost:8001/admin/instances/8/settings, and check the "Enable segments filter in assignment view" option.
   5. Go back to the dashboard and reload the page. You should see the segments dropdown now, with a total of 4.

[acelaya]

This is not part of this PR specifically, but I'm adding it to the type definition to reduce conflicts with other branches later.

[marcospri]

Nice, nothing to add here 👍