Challenge modal optimization - remove unnecessary API call - partial fix for #9371 (#9546)

* Attempt to use party data from the store rather than always fetching it from the API

* Move init code to shown() to prevent unnecessary network requests

* Use store party data in getGroup action if possible to save an API call

* Use store data rather than API call for party in challengeModal; remove unnecessary code in guilds:getGroup action

* Create party:getParty action and employ it in Group and ChallengeModal

* Use store instead of action return for party data

* Change how party data is stored

website/client/components/header/index.vue

@@ -132,7 +132,7 @@ export default {
   computed: {
     ...mapGetters({
       user: 'user:data',
-      partyMembers: 'party:members',
+      partyMembers: 'partyMembers',
     }),
 
     showHeader () {