Add CRUD operations for Programming Groups

* Correct sorting in table
* Modify page when nested in exercises
* Fix links between pages
* Link from statistics page to programming_groups/index
* Link from submission page to programming_groups/<id>
* Allow filtering for exercise ID on ProgrammingGroup#index
* Add search fields for internal and external user id on pg/index

app/views/submissions/index.html.slim

@@ -22,7 +22,7 @@ h1 = Submission.model_name.human(count: 2)
       - @submissions.each do |submission|
         tr
           td = link_to_if(submission.exercise && policy(submission.exercise).show?, submission.exercise, submission.exercise)
-          td = link_to_if(submission.contributor.is_a?(User) && policy(submission.contributor).show?, submission.contributor, submission.contributor)
+          td = link_to_if(policy(submission.contributor).show?, submission.contributor, submission.contributor)
           td = t("submissions.causes.#{submission.cause}")
           td = submission.score
           td = l(submission.created_at, format: :short)

app/views/submissions/show.html.slim

@@ -8,7 +8,7 @@
 h1 = @submission
 
 = row(label: 'submission.exercise', value: link_to_if(policy(@submission.exercise).show?, @submission.exercise, @submission.exercise))
-= row(label: 'submission.contributor', value: link_to_if(@submission.contributor.is_a?(User) && policy(@submission.contributor).show?, @submission.contributor, @submission.contributor))
+= row(label: 'submission.contributor', value: link_to_if(policy(@submission.contributor).show?, @submission.contributor, @submission.contributor))
 = row(label: 'submission.study_group', value: link_to_if(@submission.study_group.present? && policy(@submission.study_group).show?, @submission.study_group, @submission.study_group))
 = row(label: 'submission.cause', value: t("submissions.causes.#{@submission.cause}"))
 = row(label: 'submission.score', value: @submission.score)