fix(a11y): Label form fields in installer correctly

Refs #6743
fix(a11y): Use HTML5 form features on install forms

Email/Url fields are announced as such by screen readers
and prevent submission if not correctly formatted.

Required fields prevent submission of form if not filled in.
fix(a11y): Add semantic structure to installer page layout

aria landmark roles allow advanced screen reader users to
jump around the page quickly.

Aria roles are used in addition to semantic elements because
browsers do not yet consistently expose the semantic elements
as landmarks to assistive technology.

Ideally we would actually just shim this behavior with JS
so that we don't have to worry about consistently pairing
the elements with their equivalent roles.
fix(file): download adds header Content-Length