Some of the template tags for loading extension static media take an

extension and a bundle name as arguments. It's assumed that the bundle

name is always relative to the extension. When actually rendering the

bundle, we construct the full bundle ID using the relative name.
This change allows full bundle IDs to be passed to the template tags.

The given name will be checked to see if it's a full bundle ID, and if so

it will be used to render the media. If not, then we construct the full

bundle ID as before.
The motivation for this change came from a bug we found with loading review

UI extensions' static media in Review Board. Djblets' extension template

tags mostly copy what Django Pipeline does for rendering a package's static

media. Except, Djblets uses the ExtensionStaticMediaNodeMixin, which allows

us to render static media for both development setups and installed packages

of extensions. When rendering a review UI's JavaScript and CSS bundles on the

actual review UI page, we use Pipeline's javascript and stylesheet

template tags directly, which means we miss out on the nice ability to handle

installed packages.
So for example, when running a Review Board dev server with an installed

package of a review UI extension, the review UI extension's JavaScript and

CSS bundles would fail to load on the review UI page. This isn't a

problem in production servers, and isn't a problem when using a

development install of a package.
To fix this, we want to be able to use the ext_js_bundle and

ext_css_bundle template tags instead of the Pipeline ones. And to be able

to use them, we have to allow passing full bundle IDs, because that's how

JavaScript and CSS bundle names are defined on review UI extensions.
This change targets the 4.x branch because we need it to fix the bug for

Power Pack 6's review UIs (PDF and Office Document review). The bug affects

development of Power Pack 6 against Review Board 6.

Ran unit tests.
Tested using full bundle IDs with  ext_js_bundle and ext_css_bundle,

  on both RB6 and RB7 dev and production servers.
Tested using relative bundle names with  ext_js_bundle and

ext_css_bundle, on both RB6 and RB7 dev and production servers.