Added
- The settings page now lists the sites Year First is switched off on, and
switches them back on. Previously the only way back on was to visit the
site and use the toolbar popup.

Changed
- Rewritten dates are announced to screen readers as the text they replaced.
YYYY-MM-DD is a scanning aid; read aloud it is a run of digits and dashes.

Fixed
- Switching the extension, or a single site, back on no longer reloads the
page. Switching off still reloads, because a rewritten date has lost the
text it replaced.