Expand the amount of information in ExtensionResource.
ExtensionResource now lists additional information on the extension,

such as the summary, version, and related links. This will be used for

a slightly improved Manage Extensions page, and could be used for

command line management tools.
There's also an optimization thrown in for fetching the class of a

RegisteredExtension. Previously, calling get_extension_class repeatedly

would attempt to load the class mutiple times, but now it's cached.

Tested with the new Manage Extensions page.
Manually verified the contents of the API resource payloads for

enabled and disabled extensions with different links.
Unit tests pass.