Add /milestones endpoint (#8733)

Create a /milestones endpoint which basically serves as a dashboard view for milestones, very similar to the /issues or /pulls page.

Closes #8232

options/locale/locale_en-US.ini

@@ -66,6 +66,7 @@ forks = Forks
 activities = Activities
 pull_requests = Pull Requests
 issues = Issues
+milestones = Milestones
 
 cancel = Cancel
 add = Add