Upgrade Notifier

This pull request adds a message to the admin course when there are upgrades available for the WeBWorK or PG system.  It also adds a message if the OPL is newer than the library index.  (It does not tell you if there are updates for the OPL since that is updated so frequently.) 

To Test:
0) This includes changes to site.conf.dist and localOverrides.conf.dist.  So when you pull this branch you need to merge the new dist files with your existing .conf files.  
1) Create test branch in WeBWorK and PG and push the test branch to your personal repo.  Change the remote and branch variables in localOverrides.conf to be your personal repo and the test branches.  
3) Use the git web interface to make a silly change on your test branch.  When you visit the course admin page you should get a message about upgrading WeBWorK and PG.  Pull the test branch into WeBWorK and PG and the messages should go away after a reload. 
4) Use touch to change the date on the file `htdocs/DATA/library-directory-tree.json` to something old (`touch --date="Jul 2 1999" library-directory-tree.json`).  You should get a message on the admin course about upgrading the library.  Run OPL and the message should go away.