We now only create one request per icon.

In case we get a html response because of our http fallback just return
the error icon instead.

So it works in chrome too

also provies a function for css content icons (a bit hacky as it uses masks)

https://lineicons.com

It was just an experiment and doesn't work anyway on the dev server because
of missing SSL on the sentry server.

This splits things into two phases, initAssetBaseURL() will try to set
a module wide base url for all assets.

The first code to use it successfully will define the asset base url for
all future getAssetURL() calls.

This uses sentry internally, but only sends things to the server when
configured that way.

Neuber, Eugen Ramon authored 5 years ago and Reiter, Christoph committed 4 years ago

see https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest/readyState