Extension packages now make use of @beanbag/frontend-buildkit to build

static media. This includes the new support for rollup.js.
Unlike before, Djblets no longer installs just the subset of build tools

needed based on the source files. It instead installs the whole

buildkit. This simplifies the design and reduces the chance for

problems.
The new logic also replies upon build_pipeline_settings(), rather than

manually manipulating settings. This further simplifies the build logic.

Unit tests pass.
Tested building some extensions. Verified they installed

@beanbag/frontend-buildkit and built static media.