ActiveAdmin 0.6.0

Currently broken, see gregbell/active_admin#2049

[ci skip]
Give ActiveAdmin routes lower priority

Temporary workaround for bogus duplicate root route generated by
ActiveAdmin
Custom CanCan ability class for ActiveAdmin
Fix to allow ActiveAdmin comments

Still dependent on gregbell/active_admin#301.
Allow authors and editors to create admin comments
Simplifications to AdminAbility
Fixed ActiveAdmin::Comment authorization

active_admin_comment.resource_id is a string, not an integer
Use ActiveAdmin from Git until 0.6.1
Revert "Give ActiveAdmin routes lower priority"

This reverts commit c8126cbc273f972804770ed166cce4f1a664625a.
Merge pull request #30 from thetallgrassnet/activeadmin-0.6.0

ActiveAdmin 0.6.0, currently from Git until 0.6.1.