UI controls that "do something" on a page shouldn't be links. This link behaves
like a toggle to expand the uploader panel and should be a `button` element with
an aria-expanded attribute. Also:
- improves consistency with the Plugin uploader
- keeps the themes list visible when the uploader is open
- displays a notice when JavaScript is off
- adds some `hide-if-no-js` CSS classes
- removes the `themes.router.navigate()` "upload" route: seems unnecessary and breaks history
Fixes#35457.
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git-svn-id: http://core.svn.wordpress.org/trunk@37707 1a063a9b-81f0-0310-95a4-ce76da25c4cd
