Auto merge of #13358 - rjgoldsborough:form-action-url-11219, r=mbrubeck

Form action url 11219

First pass at fixing #10580.
I've added a new macro that returns a DomString with either the attr val or the doc url.
I then made the form element use that macro on the action attribute.
I also added a test that contains an iframe with a form and base url that submits to a page in
a resources directory.

I made all these changes based on https://github.com/servo/servo/pull/11219#issuecomment-223318881.

The only thing I'm confused on is how to change step 8. It looks to just be getting the action so I'm wondering if I need to change either step 9 or 10 instead?

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #10580 (github issue number if applicable).

- [X] There are tests for these changes OR

using that macro with the form action,
making the form submit process use base url,
adding tests.

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