Added join requests to Groups

• Added:
- join requests to Groups
- redux actions, reducers
- controller and functionality for fetching join requests, accepting, rejecting join requests

app/serializers/rest/group_relationship_serializer.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 class REST::GroupRelationshipSerializer < ActiveModel::Serializer
-  attributes :id, :member, :admin, :moderator
+  attributes :id, :member, :admin, :moderator, :requested
 
   def id
     object.id.to_s
@@ -19,4 +19,8 @@ class REST::GroupRelationshipSerializer < ActiveModel::Serializer
     instance_options[:relationships].moderator[object.id] ? true : false
   end
 
+  def requested
+    instance_options[:relationships].requested[object.id] ? true : false
+  end
+
 end