Add option to exclude device reports in form forwarding

Addresses this ticket: http://manage.dimagi.com/default.asp?121924#702771

The couch views and forms for form forwarding, case forwarding, form stub forwarding, etc. were all the same class. To add the check box to just the form forwarding form, I subclassed the `GenericRepeaterForm` and the `AddRepeaterView` classes. This also required corresponding changes to the urls file and `hqwebapp/models.py`, which controls the side navigation bar appearance. 
Could simplify this condition to use `getattr(repeater, 'exclude_device_reports', False)` ... at least I think the default should be `False`. Let's see if that's right (assuming `is_device_report(payload)` is always true in these examples)...

attribute value | `not (hasattr(...) and ... and ...)` | `not (getattr(...) and ...)` | result
-----|-----|-----|-----
missing | `not (False and ... and ...)` | `not (False and ...)` | `True`
`True` | `not (True and True and True)` | `not (True and True)` | `False`
`False` | `not (True and False and True)` | `not (False and True)` | `True`