Issue #197: Replace theme_checkboxes and theme_radios with theme_container

This PR for https://github.com/backdrop/backdrop-issues/issues/197 combines both of the issues for replacing checkboxes and radios into a single PR. Considering they both have identical implementations and depend on the same changes, I've combined them into a single issue.

This PR replaces both the previous PRs: https://github.com/backdrop/backdrop/pull/266 and https://github.com/backdrop/backdrop/pull/319.