install-theme: install a theme from WordPress.org, by slug #350

New tool saddle/install-theme: the same as install-plugin (#349) for themes, with themes_api(), Theme_Upgrader::install() and the install_themes capability.

Extra refusal: a child theme whose parent theme is not installed. Core's installer would otherwise fetch and install the parent on its own, which nobody previewed.

Acceptance: the same test matrix as install-plugin, plus the child-theme refusal and a run-time guard test.