Add usercontent template tag to hide data

On the user profile page we need to hide the personal data and edit
controls from the public but show them to the logged in user.

The main benefit of the tag is to make template code that shows
non-public data to only one specific user more self documenting.
Rename audit_extras to auth_extras

Both template tags deal with authorization rather than auditing.
Merge pull request #196 from jwalgran/topic/usercontent-tag

Add usercontent template tag to hide data