Fix "Not Participating" filter checkbox on Discover Challenges screen, Fixes #13025 (#13183)

* change 'role' options and filters to 'membership'

* added membership filtering function to discover challenges

* fix accidental deletion

* complete removal of roles from challenge filters

* abstract challenge "participating" filtering into mixin

* selecting "participating" and "not participating" shows all challenges

website/client/src/components/challenges/findChallenges.vue

@@ -31,7 +31,8 @@
       </div>
       <div class="row">
         <div
-          v-if="!loading && filteredChallenges.length === 0"
+          v-if="!loading &&
+          this.filteredChallenges.length === 0"
           class="no-challenges text-center col-md-6 offset-3"
         >
           <h2 v-once>
@@ -41,7 +42,7 @@
       </div>
       <div class="row">
         <div
-          v-for="challenge in filteredChallenges"
+          v-for="challenge in this.filteredChallenges"
           :key="challenge._id"
           class="col-12 col-md-6"
         >
@@ -170,9 +171,6 @@ export default {
   },
   computed: {
     ...mapState({ user: 'user.data' }),
-    filteredChallenges () {
-      return this.challenges;
-    },
   },
   mounted () {
     this.$store.dispatch('common:setTitle', {