Adding support for home pages that immediately redirect to other sites.

Some sites would like to use their own home pages for workshops rather than
GitHub pages.  However, all of our website machinery expects to harvest workshop
information from GitHub.  The (hacked) solution is to allow people to create a
GitHub repository containing a home page that immediately redirects to their own
site.  To do this, we add a new field at `index.html` YAML header called
redirect that store the address of the workshop's home page.
Merge pull request #764 from r-gaia-cs/adding-redirect-workshop-page

Adding support for home pages that immediately redirect to other sites. (2)