Fix Vue warnings about string idKey in related issues

```
[Vue warn]: Invalid prop: type check failed for prop "idKey". Expected
Number, got String.
```

We now fail on Vue warnings,
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12163/

spec/javascripts/issuable/related_issues/components/add_issuable_form_spec.js

@@ -3,7 +3,7 @@ import eventHub from '~/issuable/related_issues/event_hub';
 import addIssuableForm from '~/issuable/related_issues/components/add_issuable_form.vue';
 
 const issuable1 = {
-  id: '200',
+  id: 200,
   reference: 'foo/bar#123',
   displayReference: '#123',
   title: 'some title',
@@ -12,7 +12,7 @@ const issuable1 = {
 };
 
 const issuable2 = {
-  id: '201',
+  id: 201,
   reference: 'foo/bar#124',
   displayReference: '#124',
   title: 'some other thing',

spec/javascripts/issuable/related_issues/components/related_issues_block_spec.js

@@ -3,7 +3,7 @@ import eventHub from '~/issuable/related_issues/event_hub';
 import relatedIssuesBlock from '~/issuable/related_issues/components/related_issues_block.vue';
 
 const issuable1 = {
-  id: '200',
+  id: 200,
   reference: 'foo/bar#123',
   displayReference: '#123',
   title: 'some title',
@@ -12,7 +12,7 @@ const issuable1 = {
 };
 
 const issuable2 = {
-  id: '201',
+  id: 201,
   reference: 'foo/bar#124',
   displayReference: '#124',
   title: 'some other thing',

spec/javascripts/issuable/related_issues/components/related_issues_root_spec.js

@@ -8,7 +8,7 @@ const defaultProps = {
 };
 
 const issuable1 = {
-  id: '200',
+  id: 200,
   reference: 'foo/bar#123',
   title: 'issue1',
   path: '/foo/bar/issues/123',
@@ -17,7 +17,7 @@ const issuable1 = {
 };
 
 const issuable2 = {
-  id: '201',
+  id: 201,
   reference: 'foo/bar#124',
   title: 'issue1',
   path: '/foo/bar/issues/124',

spec/javascripts/issuable/related_issues/services/related_issues_service_spec.js

@@ -3,6 +3,7 @@ import Vue from 'vue';
 import RelatedIssuesService from '~/issuable/related_issues/services/related_issues_service';
 
 const issuable1 = {
+  id: 200,
   reference: 'foo/bar#123',
   title: 'some title',
   path: '/foo/bar/issues/123',

spec/javascripts/issuable/related_issues/stores/related_issues_store_spec.js

 import RelatedIssuesStore from '~/issuable/related_issues/stores/related_issues_store';
 
 const issuable1 = {
-  id: '200',
+  id: 200,
   reference: 'foo/bar#123',
   title: 'issue1',
   path: '/foo/bar/issues/123',
@@ -10,7 +10,7 @@ const issuable1 = {
 };
 
 const issuable2 = {
-  id: '201',
+  id: 201,
   reference: 'foo/bar#124',
   title: 'issue1',
   path: '/foo/bar/issues/124',