Mostly closes #115, commentless feedback now filterable

core/serializers/subscription.py

core/views/subscription.py

frontend/src/api.js

...nd/src/components/feedback_list/FeedbackSearchOptions.vue

frontend/src/components/feedback_list/FeedbackTable.vue

frontend/src/pages/base/FeedbackHistoryPage.vue

...rc/store/modules/feedback_list/feedback-search-options.js

frontend/src/store/modules/feedback_list/feedback-table.js

frontend/src/store/store.js

frontend/src/util/helpers.js