Code Cleanup: Usage of Polymorphic User instead of ExternalUser and InternalUser

Renamed requestor_user_id -> user_id
Index of request_for_comments now returns the n (5?) newest requests per user. Solved via sliding windows in postgres, code added to model.
Added route to /my_request_for_comments/ that shows all requests for the current user.
Changed view from ERB to slim

app/views/request_for_comments/index.html.slim

@@ -0,0 +1,19 @@
+h1 = RequestForComment.model_name.human(count: 2)
+
+.table-responsive
+  table.table
+    thead
+      tr
+        th = t('activerecord.attributes.request_for_comments.exercise')
+        th = t('activerecord.attributes.request_for_comments.execution_environment')
+        th = t('activerecord.attributes.request_for_comments.username')
+        th = t('activerecord.attributes.request_for_comments.requested_at')
+    tbody
+      - @request_for_comments.each do |request_for_comment|
+        tr data-id=request_for_comment.id
+          td = link_to(request_for_comment.exercise.title, request_for_comment)
+          td = request_for_comment.exercise.execution_environment
+          td = request_for_comment.user.name
+          td = request_for_comment.requested_at
+
+= render('shared/pagination', collection: @request_for_comments)