Feature/embed load css

Properly loads CSS and images for embedded exhibits.  This follows from #72.  The resolution to the issue described there was to move the require config out of the static main file and duplicate it in the build file and in a variable in the templated show.js file.  Finally, the 'mainConfigFile' setting needed to be unset.  I've tested this both in a development and production environment.

This also includes a modification to 500.html so it doesn't inherit form site_base.html.  The use of variables that require a request context was causing it to mask other errors.