Load css and js from CDN

[Flix6x]

What I did:

• Load external js from CDN.
• Load most external css from CDN. There's just one I customized to not conflict with the rest of our app (rq-dashboard-bootstrap.min.css), another I can't find hosted on a CDN (weather-icons.min.css), and finally one that I moved out of the external css folder, because it just contains a customisable skin configuration for our range slider (ion.rangeSlider.skinCustom.css).

I read that, if CDN-based scripts were not loaded from CDN for whatever reason, you should fallback to a local copy. Is that functionality you would like to have?

[nhoening]

I haven't thought about the fallback yet. I guess the recommendation is easier for closed-source projects, as they don't have to consider licensing :D

So there are three resources which we'll definitely keep in the source, so we'll mention them in NOTICES?

[Flix6x]

As you pointed out earlier, these js and css files usually have very permissive licences. I don't think it matters from where they are served, from a copyright perspective.

I made sure that all our css and js dependencies have permissive licences: all MIT, except Bokeh (BSD-3), Leaflet and rq-dashboard (both BSD-2). I could include a full list in the NOTICES file, if you'd like.

[Flix6x]

I upgraded external css and js, and by doing so I was also able to remove one more css file: an ion.rangeslider skin that is now supported out of the box. We only use it on the control page (currently hidden), but it looks quite nice.

The daterangepicker also looks nicer now, with users being nudged to use one of the default date ranges rather than set a custom date range.

Otherwise, no noticeable changes as far as I can tell.