Merge branch 'fix-oxford-comma' into 'master'

Add a missing Oxford comma in some text on the Activity graph.

Resolves #14668. [ci skip]

See merge request !3625

app/views/users/calendar.html.haml

@@ -7,4 +7,4 @@
       '#{user_calendar_activities_path}'
     );
 
-.calendar-hint Summary of issues, merge requests and push events
+.calendar-hint Summary of issues, merge requests, and push events