2. Add site configuration overrides to theming/helpers.py
3. Move microsite.get_value from theming/helpers to site_configuration/helpers
4. Move microsite_configuration.microsite.get_value usages to site_configuration.helpers.values
2. Update COMPREHNSIVE_THEME_DIR to COMPREHENSIVE_THEME_DIRS
3. Update paver commands to support multi theme dirs
4. Updating template loaders
5. Add ENABLE_COMPREHENSIVE_THEMING flag to enable or disable theming via settings
6. Update tests
7. Add backward compatibility for COMPREHEHNSIVE_THEME_DIR
* mattdrayer: Add helpers.get_value test
* mattdrayer: Change to simpler implementation, per @douglashall
* mattdrayer: Address quality violations and test failures
Comprehensive theming did not work with django templates (used by course wiki).
The reason it didn't work was that in order for the theme to work, theme template folder
has to be added to django template dirs setting *before* django startup.
After django startup, modifying `settings.DEFAULT_TEMPLATE_ENGINE['DIRS']` has no effect,
because at that point the template engine is already initialized with a copy of the
template dirs list.
Instead of running the theme startup code as an autostartup hook, we manually run it
*before* `django.setup()`. This is fine because theme startup code doesn't have to do
anything else besides modifying some settings and doesn't actually need django to be
initialized.
