Cache on a per-frontend-module basis

• Remove the full_page_cache config option and all related functionality
• Deprecate the notion of “dynamic” vs. “static” frontend modules
• Default old dynamic modules to not cache and old static modules to cache by subpage
Introduce UserError

Allows exceptions to be thrown and not caught and still not mailed to the user

Fixes #143
Merge branch 'frontend_module_output_caching'

* frontend_module_output_caching:
  Introduce UserError
  Cache on a per-frontend-module basis