create GetHeroParty API v3 route to fetch a given user's party data

Only some data from the party will be loaded (e.g., not private
data such as name, description).

Includes tests for the route.

See the next commit for front-end changes that use this.

website/common/script/errors/apiErrorMessages.js

@@ -14,6 +14,7 @@ export default {
   guildsOnlyPaginate: 'Only public guilds support pagination.',
   guildsPaginateBooleanString: 'req.query.paginate must be a boolean string.',
   groupIdRequired: 'req.params.groupId must contain a groupId.',
+  groupWithIDNotFound: 'Group with id "<%= groupId %>" not found.',
   groupRemainOrLeaveChallenges: 'req.query.keep must be either "remain-in-challenges" or "leave-challenges"',
   managerIdRequired: 'req.body.managerId must contain a User ID.',
   noSudoAccess: 'You don\'t have sudo access.',