Create activities table
Record submission create activity
Record contest create/update activity
Record tag create activity
Record tags update activity
contest_create activity must be created in controller

The user who updates the contest != owner.
List recent activities
Render tag activity in problem page
Render contest activities in contest page
Render activities in user page
Render created_at of activity
Record contest join/leave activities
Merge pull request #6 from eagletmt/activity-feed

Implement activity feed