Expose subscribed attribute for epics on API

Expose if user is subscribed to an epic when getting a
single epic from API.

changelogs/unreleased/issue_11240.yml

+---
+title: Expose subscribed attribute for epic on API
+merge_request: 18475
+author:
+type: added

doc/api/epics.md

@@ -147,7 +147,8 @@ Example response:
   "closed_at": "2018-08-18T12:22:05.239Z",
   "labels": [],
   "upvotes": 4,
-  "downvotes": 0
+  "downvotes": 0,
+  "subscribed": true
 }
 ```
 

ee/lib/api/epics.rb

@@ -54,7 +54,9 @@ module API
       get ':id/(-/)epics/:epic_iid' do
         authorize_can_read!
 
-        present epic, with: EE::API::Entities::Epic, user: current_user
+        present epic, options, user: current_user,
+                               with: EE::API::Entities::Epic,
+                               include_subscribed: true
       end
 
       desc 'Create a new epic' do

ee/lib/ee/api/entities.rb

@@ -323,6 +323,15 @@ module EE
           end
         end
 
+        # Calculating the value of subscribed field triggers Markdown
+        # processing. We can't do that for multiple epics
+        # requests in a single API request.
+        expose :subscribed, if: -> (_, options) { options.fetch(:include_subscribed, false) } do |epic, options|
+          user = options[:user]
+
+          user.present? ? epic.subscribed?(user) : false
+        end
+
         def web_url
           ::Gitlab::Routing.url_helpers.group_epic_url(object.group, object)
         end

ee/spec/fixtures/api/schemas/public_api/v4/epic.json

@@ -42,7 +42,8 @@
     "closed_at": { "type": ["string", "null"] },
     "web_edit_url": { "type":  "string" },
     "web_url": { "type":  "string" },
-    "reference": { "type":  "string" }
+    "reference": { "type":  "string" },
+    "subscribed": { "type": ["boolean", "null"] }
   },
   "required": [
     "id", "iid", "group_id", "title"

ee/spec/requests/api/epics_spec.rb

@@ -422,6 +422,12 @@ describe API::Epics do
         expect(response).to match_response_schema('public_api/v4/epic', dir: 'ee')
       end
 
+      it 'exposes subscribed field' do
+        get api(url, epic.author)
+
+        expect(json_response['subscribed']).to eq(true)
+      end
+
       it 'exposes closed_at attribute' do
         epic.close