Image optimization is fairly slow (over a minute), and isn't necessary for test or development builds. It is now only run as part of the build gulp task, which is used during gulp dist.

A few unused gulp tasks have been removed as well. There were two high-level tasks and one style formatting task that were not used by any npm scripts, so were probably unused generally.

The dev task was a duplcate of dev:extension. The build:extension task was useful for just building the extension without performing other steps required by the final production bundle, but it was broken. It didn't correctly build the ui-libs and bg-libs required. Instead of fixing it, it has been removed until the handling of those separate library bundles is simplified.

The style formatting task seems like it could be useful, but I'm unsure about keeping it around as opt-in, as in practice it'll just end up being ignored. Moreover the library authors themselves are recommending switching to prettier, so I think we're better off removing it for now, then considering using prettier if we want to introduce something like this again.

The stylelint dependency was added because it's a peer dependency of gulp-stylelint that should have already been listed among our dependencies. It hadn't caused a problem before because it happened to be a transitive dependency of gulp-stylefmt, which is no longer needed and has been removed.