Fix datagrid pagination on Django 1.5.

Django 1.5 didn't like how we were attempting to access a variable in
the context that didn't exist, and would fail with a traceback.

We now guarantee that these variables always exist.
Release Djblets 0.7.20.
Bump to 0.7.21 dev.
Add a new has_list_access_permissions function.

This is used to determine if a user can have access to the resource
list. It makes it easier to add custom logic beyond just using
model_parent_key, much like the other permission functions.
Merge branch 'release-0.6.x' into release-0.7.x
Merge branch 'release-0.7.x'