fix: use DOM base href for web XHRs

[JVMerkle]

By using html.document.baseUri.toString() as base URL for the web-app, the base tag in the DOM is considered correctly.
This allows serving the application from a sub-directory (e.g. https://foo.com/kitchenowl/).

Tested with and without subdir-serving.

Future todo's: Replace DOM <base href="..."> with environment variable content etc. for easy containerized deployment.

Partial implementation for #130, #237

[JVMerkle]

Hi @TomBursch,
using conditional imports the other platforms seem to work fine. Obviously the structure is not pretty nice, any suggestions regarding location/naming of the dart:html wrapper? (see latest commit)
Please note I have no idea of dart, conditional imports might not be the way to go here.

[TomBursch]

Hi @JVMerkle, thanks for the PR! Conditional imports are fine. I think putting the code into lib/platform/dart_html/ would be better, or what do you think? But other than that, I'm happy with it.

[JVMerkle]

Hi @TomBursch, lib/platform sounds nice. I am also preparing another MR where the base can be controlled with BASE_HREF. BR, Julian