Resolve rendered URLs according to the provided <base>

This is done by using the computed property value rather than the
literal attribute value when rendering href and src attributes.
Helpfully, this provides perfect URL resolution natively in the browser,
which means the document's <base> is respected and all that good stuff.

For GeneratedContentNodes, we also need to find all DOM elements inside
the rendered DOM that have href or src attributes and resolve those.
This is done in the new getRenderedDomElements() function, which the
existing cleanup steps (remove <link>/<meta>/<style>, clone for
correct document) were moved into.

In order to make sure that the computed values are always computed
correctly, we need to make sure that in cases where HTML strings
in data-mw are parsed, they're parsed in the context of the correct
document so the correct <base> is applied.

We still need to solve this problem for models that actually store and
edit an href or src as an attribute. I'll post more about that on
bug 48915.

Bug: 48915
Change-Id: Iaccb9e3fc05cd151a0f5e632c8d3bd3568735309
Merge "Resolve rendered URLs according to the provided <base>"