Add disambiguation button.

- Replaces the hatnote at the top of a page with a styled button.
- If a page has more than one hatnote, the other hatnotes are hidden.
- If the hatnote has more than one link, the *last* link is used as the
  disambiguation link (in accordance with the Style Guide).

Change-Id: I0b38a101f642b1c72b46fd249f11ebe9826ac85c