Adjust the way that we override HttpResponse.content.

The WebAPIResponse.content property overrides Django's HttpResponse.content in
kind of a hacky way, using private APIs that have gone away in Django 1.5. This
change overrides the property by proxying back in to the content property in
the super class. This is kind of awkward because of the way that properties
work, but the method herein seems to work and is much less gross than the
previous approach.

Testing done: Ran unit tests
Reviewed at https://reviews.reviewboard.org/r/4583/
Don't blindly call next_page_number() or previous_page_number().

The pagination methods have been changed to raise EmptyPageError if there's no
such page. We therefore can't blindly add things to the context, but rather
should check that the pages exist before we call these methods.

Testing done:
* Loaded the Review Board dashboard.
* Ran unit tests.

Reviewed at https://reviews.reviewboard.org/r/4587/
Default to no indentation for the json templatetag.

The json_dumps templatetag has an "indent" parameter, which defaulted to '0'.
This caused the generated output to include newlines, but indent things zero
spaces. If we default this to None, it will skip all extraneous whitespace.

Testing done:
Used this in a future change, and saw that it now dumped my big data blob onto
a single line in the output rather than formatting it super ugly.

Reviewed at https://reviews.reviewboard.org/r/4605/
Change django.utils.simplejson to json.

Django 1.5 deprecates the use of the bundled simplejson, because Python 2.6's
json module includes everything that's needed.

Testing done: Ran djblets and Review Board unit tests.
Reviewed at https://reviews.reviewboard.org/r/4604/
Increased z-index on modal-box.

A prior change to Review Board made it so the user account navigation pop-up
was showing over a modal-box. Increase the z-index to put everything on the
right layer.

Testing done:
Clicked on a review request, hit review which brought up the modal screen and
ensured that the usernav drop down was no longer visible over the modal box.

Reviewed at https://reviews.reviewboard.org/r/4607/
Import ValidationError from django.core.exceptions instead of django.forms.

They haven't announced any official deprecation of this, but it seems like
exceptions are being consolidated in one place.

Testing done: Ran unit tests
Reviewed at https://reviews.reviewboard.org/r/4609/
Bump djblets to 0.8

Version bump in preparation for requiring Django 1.5.x
Fix WebAPIError.with_message, and add WebAPIError.with_override.

WebAPIError.with_message modified the main instance of an error, instead
of creating a copy. This led to pollution of the error instances. In
practice, Djblets and Review Board aren't bit by it, by this may change
over time. Now it creates a copy.

Since it's also useful to override headers, a new with_override function
has been added that takes message and headers. with_message now just
wraps this.