ARIA common files approach

In today's ARIA Editors call we discussed the ongoing issue with 
handling the common resources when splitting the ARIA repository:

https://www.w3.org/2016/12/07-aria-editors-minutes.html#item03

Given intractable problems with getting submodules to work with rawgit, 
and the need for that feature to work, we revisited forking. The 
proposal now is to:

 1. Put the common files in their own repository (aria-common) as
    previously planned;
 2. Put copies of the files (forks) in each of the ARIA repositories
    after we split;
 3. Set up a commit hook that updates each of the forks whenever an
    update is pushed to aria-common;
 4. Document that people should not edit the aria-common forks in repos.

This isn't the theoretically right way to use git but is practical and 
achievable, and unblocks the repository split project. We wanted to run 
the thought past the rest of the editors to see about thoughts before 
making a firm decision to implement.

Thoughts?

Michael