This updates our JavaScript bundling process in Review Board to use the

new @beanbag/frontend-buildkit Node package, bringing with it

rollup.js as a JavaScript bundler. This allows us to use modern

JavaScript module support, which can help us better organize our code

and move away from setting everything directly in the RB namespace.
Ultimately, anything exported by our bundles will be set in the RB

namespace, but this is now controlled at bundle-time instead of being

assumed in code.
Configuration files for Babel, Rollup, ESLint, and TypeScript have been

set up to ensure we have a working build configuration. These specify

include paths, mappings of import paths to global namespaces (like

djblets/* to Djblets), and lint rules.
A .djblets directory is now set up during setup.py develop to point

to the Djblets package directory, to ensure static media paths to

Djblets are correct. It will be required to run setup.py develop once

after this change, and any time the target Djblets has moved/changed.
Going forward, the plan is to work toward each Pipeline bundle mapping

to a single index.ts file, which would then import and re-export

anything that should be public. This would give rollup.js full control

of all JavaScript bundling, and let Pipeline take that result and make

it compatible with the Django static media system.
In the meantime, we'll continue to have plenty of legacy JavaScript

sitting around modifying RB directly. This is fine, and will not

conflict. We'll need to be thoughtful in migrating this code. We don't

want to just rename and add exports. Instead, for many of these, we

should think about where the modules should truly live first.
A few modules did get updated in this change, due to necessity.

rb/js/common/index.ts was introduced to set some default exports

(Product and EnabledFeatures), so that we could guarantee that RB

was set to something by default during bundle. This replaces the old

window.RB = {} line. Similarly, some nested namespaces have been

exported by other modules (for, e.g., RB.Admin).

Set up some imports/exports between our various bundles, and importing

from Djblets. Verified that the generated code in the browser was all

correct, and that I could package up the resulting JavaScript.
Tested datagrid pages, review request pages, admin UI pages, and the

My Account pages for errors.