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/models/group_join_request.rb

@@ -0,0 +1,17 @@
+# == Schema Information
+#
+# Table name: group_join_requests
+#
+#  id         :bigint(8)        not null, primary key
+#  account_id :bigint(8)        not null
+#  group_id   :bigint(8)        not null
+#  created_at :datetime         not null
+#  updated_at :datetime         not null
+#
+
+class GroupJoinRequest < ApplicationRecord
+  belongs_to :group
+  belongs_to :account
+
+  validates :account_id, uniqueness: { scope: :group_id }
+end