Rewrite build system away from gulpjs

[jniles]

Gulpjs hasn't seen a release in four years, and we are starting to see more and more errors pop up due to advisories in the underlying codebase. Although Gulp v5 is ostensibly coming, there is no commit activity in the repository to show for it.

We ought to consider moving to something else. I would be a proponent of using Webpack which has been successful to date and actively maintained. It would also present an opportunity for us to look at the relationship between the bhima server and client and potentially move them to different repositories (initially proposed in #6228).

[jmcameron]

Note that there has been a recent update for gulpjs. It includes a lot of breaking changes, so it seems prudent to wait for a few releases for it to settle down before updating our builds to deal with the changes in gulp.