update to terser v5
replace gulp-remove-code with a rough equivalent in the gulpscript itself
replace gulp-inline with gulp-inline-source-html
remove base64 modules as redundant when running inline-source (does both)
remove crass as redundant when running inline-source (uses csso)
fixup jquery source map comment (but still keep the actual file)

at least now this seems stable enough to start more of the ui tweaking

support es6 syntax
use js comment style in custom.js
only minify the custom.js

using 4.x branch, since 5.x wants async / await and more changes to the gulp script

Bump npm version installed by travis
Update package-lock for latest npm version
Bump packages versions and clean-up unused
Missing datatables.js images to avoid warnings