Make theme handling consistent with plugins

[galbaras]

For some reason, the user interface for themes is very different from the one used for plugins.

Specifically, theme changelog links only ever appear on the theme detail page and nowhere else, and relying on Theme URI to figure out where the changelog is often incorrect, including for recent core themes.

Since there's already much of the codebase for this in the Plugins area, bring Themes inline with this shouldn't be super difficult, and will make the admin area more consistent, while also enabling better usability for external management systems.

This can be facilitates by a header field, e.g. Changelog URI or Update URI + an assumed readme.txt, possibly coupled with a filter hook.