Revamp the logic for displaying the sharedEdit button

The problem before is that while staff members could see the button, the other
user who was supposed to be able to see it -- the post's original author, could
not. Now, we check if the user is an admin, or if the post in question is
theirs.
Merge pull request #2 from evaryont/master

Revamp the logic for displaying the sharedEdit button