Removed exam and student subscriptions for tutors (Frontend)

Also cleaned up the subscription code considerably

frontend/src/components/subscriptions/Subscription.vue

+<template>
+  <v-list-tile
+    exact
+    :to="{name: 'subscription', params: {pk: subscriptionPk}}"
+  >
+    <v-list-tile-content class="ml-3">
+      {{query_key ? query_key : 'Active'}}
+    </v-list-tile-content>
+    <v-list-tile-action-text>
+      {{stageMap[feedback_stage]}}
+    </v-list-tile-action-text>
+  </v-list-tile>
+</template>
+
+<script>
+  export default {
+    name: 'subscription',
+    props: {
+      subscriptionPk: {
+        types: String,
+        required: true
+      },
+      feedback_stage: {
+        type: String,
+        required: true
+      },
+      query_key: {
+        type: String
+      }
+    },
+    data () {
+      return {
+        stageMap: {
+          'feedback-creation': 'create',
+          'feedback-validation': 'validate',
+          'feedback-conflict-resolution': 'conflict'
+        }
+      }
+    }
+  }
+</script>
+
+<style scoped>
+
+</style>