Add notification unsubscribe redirect path for epics

app/controllers/sent_notifications_controller.rb

 class SentNotificationsController < ApplicationController
+  prepend EE::SentNotificationsController
+
   skip_before_action :authenticate_user!
 
   def unsubscribe
@@ -17,16 +19,20 @@ class SentNotificationsController < ApplicationController
     flash[:notice] = "You have been unsubscribed from this thread."
 
     if current_user
-      case noteable
-      when Issue
-        redirect_to issue_path(noteable)
-      when MergeRequest
-        redirect_to merge_request_path(noteable)
-      else
-        redirect_to root_path
-      end
+      redirect_to noteable_path(noteable)
     else
       redirect_to new_user_session_path
     end
   end
+
+  def noteable_path(noteable)
+    case noteable
+    when Issue
+      issue_path(noteable)
+    when MergeRequest
+      merge_request_path(noteable)
+    else
+      root_path
+    end
+  end
 end

ee/app/controllers/ee/sent_notifications_controller.rb

+module EE
+  module SentNotificationsController
+    extend ::Gitlab::Utils::Override
+
+    private
+
+    override :noteable_path
+    def noteable_path(noteable)
+      return epic_path(noteable) if noteable.is_a?(Epic)
+
+      super
+    end
+  end
+end

ee/spec/controllers/ee/sent_notifications_controller_spec.rb

+require 'rails_helper'
+
+describe SentNotificationsController do
+  let(:user) { create(:user) }
+
+  context 'Unsubscribing from an epic' do
+    let(:epic) do
+      create(:epic, author: user) do |epic|
+        epic.subscriptions.create(user: user, project: nil, subscribed: true)
+      end
+    end
+    let(:sent_notification) { create(:sent_notification, project: nil, noteable: epic, recipient: user) }
+
+    before do
+      sign_in(user)
+      get(:unsubscribe, id: sent_notification.reply_key)
+    end
+
+    it 'unsubscribes the user' do
+      expect(epic.subscribed?(user)).to be_falsey
+    end
+
+    it 'sets the flash message' do
+      expect(controller).to set_flash[:notice].to(/unsubscribed/)
+    end
+
+    it 'redirects to the merge request page' do
+      expect(response)
+        .to redirect_to(group_epic_path(epic.group, epic))
+    end
+  end
+end