Fix issue with submissions list count variable

Most notably fixes "Show all submissions" button missing.

With the new pseudonymization feature, in exercise/staff_views:ListSubmissionsView,
the submissions queryset is iterated over, resulting in evaluating the queryset,
e.g. it being converted into a list which has no .count() method. Fix by including
the count variable in the template context.

exercise/staff_views.py

@@ -80,7 +80,8 @@ def get_common_objects(self) -> None:
         self.all = self.request.GET.get('all', None)
         self.all_url = self.exercise.get_submission_list_url() + "?all=yes"
         self.submissions = qs if self.all else qs[:self.default_limit]
-        self.note("all", "all_url", "submissions", "default_limit")
+        self.count = len(self.submissions)
+        self.note("all", "all_url", "submissions", "default_limit", "count")
 
 
 class SubmissionsSummaryView(ExerciseBaseView):

exercise/templates/exercise/staff/_submissions_table.html

@@ -2,7 +2,6 @@
 {% load course %}
 {% load exercise %}
 {% load static %}
-{% with count=submissions.count %}
 
 <div class="clearfix">
 	<div class="pull-right">
@@ -121,5 +120,3 @@
 </table>
 
 {% include "exercise/staff/_regrade_submissions.html" %}
-
-{% endwith %}