Feature/2119 redirect users if exam mode is on

This is for #2119 - Redirect users if exam mode is on on that device

Hi @aronasorman - I've already added `EXAM_MODE_ON` boolean at `fle_utils.config.models.Settings` on my branch and created the `student_testing.middleware.ExamModeCheck` middleware.

However I need to verify these:

1. When you say user profile page, is this the `reverse("account_management")`?
2. How do I know the logged-in user is a student?  I checked by logging-in as a student but `request.user.is_authenticated()` returns an `AnonymousUser`, I'm using the `request.is_logged_in` for now and used `request.is_admin` and `request.is_teacher` to bypass the redirect.
3. How do we prevent redirect loops?  I saw GET calls to `/api/*` and `/securesync/*` which redirects to the redirect url causing a redirect loop - so I put an ignore list like

    ```python
    redirect_to = reverse("account_management")
    url_exceptions = [
        "^/api/*",
        "^/securesync/*",
        redirect_to
    ]

This works when a FacilityUser logs-in so far.

I looked at #2122 also to cross-check.  When a specific exam is flagged, the students will be redirected to the proper exam.  I just added a TODO: comment at the bottom where we need to put the codes for this.