Removed loading of custom verbatim tag

Django prior to 1.5 had no verbatim tag, so Redwood was using a custom implementation. Since Redwood has been upgraded to Django 1.7, we can take advantage of the more flexible built-in verbatim tag.

The built-in verbatim tag is loaded automatically. {% load verbatim %} is currently being used to load the custom verbatim implementation. Since many of the example experiments still use this, expecon/templates/templatetags/verbatim.py has not been deleted (yet).

Commit text:

- removed {% load verbatim %} from default experiment pages.
- however, the custom verbatim tag (expecon/templates/templatetags/verbatim.py) is still present for compatibility with experiments which still try to load it.