This switches Djblets over to the new @beanbag/frontend-buildkit

dependency, which defines a standard set of dependencies for building

both CSS and JavaScript media.
The new buildkit dependency includes Rollup. This is now enabled,

allowing us to begin introducing JavaScript (TypeScript, specifically)

modules that can import other modules.
To start off, most (though not all) of our Pipeline JavaScript bundles

now include an index.ts, which can be populated with imports as we go

forward. The Pipeline bundles separate these out from the legacy

JavaScript via comments. There's also a rollup.config.js to tell

Rollup how we want to bundle things, which will be shared for each

Pipeline bundle.
It'll be the responsibility of each index.ts to import any modules

needed, and to set them in a Djblets namespace.
Extension building has also been updated to work with this.

Temporarily converted some code to use Rollup. This involved converting

some existing files (both .ts and .es6.js) to be modules that

exported content instead of setting Djblets.*explicitly. Re-exported

in an index.ts and verified the resulting bundled file set those on

Djblets.
Then updated an index.ts in a different bundle, imported content from

the other bundle, and exported new code based on that. Verified it

properly accessed the imported content from Djblets and exported back

to Djblets without overwriting anything.
Compiled Djblets static media for packaging, and manually inspected

the files, making sure this all looked correct.
Built the static media in Review Board by browsing pages, and verified

it used Djblets's rollup.config.js, .babelrc, and tsconfig.json for

the builds, instead of Review Board's (tested with and without Review Board

providing those files). Verified Review Board was able to successfully use

the modified code that exported some functionality, and that some dummy

objects I set up and modified via the exports were accessible in Djblets.