Switch to using gulp-better-rollup

Means our rollup bundling doesn't leave any
artefact files lying around that we'd then have to
deal with.

Also includes:
- removal of some JSHint config' marking the
  artefacts as scripts to ignore
- use of streamqueue package to allow the same
  ordering of scripts as before

package.json

@@ -30,6 +30,7 @@
     "gulp-add-src": "1.0.0",
     "gulp-babel": "8.0.0",
     "gulp-base64-inline": "1.0.4",
+    "gulp-better-rollup": "4.0.1",
     "gulp-concat": "2.6.1",
     "gulp-include": "2.3.1",
     "gulp-sass": "4.0.2",
@@ -38,6 +39,7 @@
     "jquery": "3.4.1",
     "query-command-supported": "1.0.0",
     "rollup": "1.23.1",
+    "streamqueue": "1.1.2",
     "textarea-caret": "3.1.0",
     "timeago": "1.6.5"
   },