Update imports to use the new djblets.webapi modules.

In /r/5899/, djblets.webapi.core was split into different modules. This
updates the Review Board imports to use the new modules, instead of
relying on the legacy core module.

Reviewed at https://reviews.reviewboard.org/r/5900/
Merge branch 'release-2.0.x'
Set a default for settings.AJAX_SERIAL when running unit tests.

The change to prevent extension reloading during tests needed an
accompanying trivial change to set settings.AJAX_SERIAL. This wasn't set
before, and unit tests depended on it. We were getting a working value,
though, because ExtensionManager recalculates it when loading
extensions.
Upgrade jquery.cookie to 1.4.1.

This is because of Bug 3387 where if any cookie exists on the domain that has
an invalid URL escaping, for example, a cookie like 'foo=%%', then the entire
site will roughly stop working. This was fixed upstream in jquery.cookie
between 1.3.1 and 1.4.1.

This can be verified by running the live 2.0.1 on a machine, and running the
following on a JS console:

```javascript
document.cookie = 'foo=%%';
```

and then refreshing the page. it will stop working; if you try to view a diff,
the parts will not load but will spin forever. This seems to be because the JS
fails to load. After applying this diff it will start working :). This
situation can occur if reviewboard is run on a subdomain like `rb.example.com`
and there are cookies on `example.com` that contain this kind of string. This
is a regression from 1.7

Testing Done:
Played around with the site and didn't see anything wrong. More testing would
be appreciated as this is my first review posted and whatnot. I'm just going to
roll this out internally to our site though and hope for the best :)

Bugs closed: 3387

Reviewed at https://reviews.reviewboard.org/r/5888/
Merge branch 'release-2.0.x'