This makes Discourse compliant with latest Google PWA requirements,
so we get the App Install banner back.
Should bump our Lighthouse PWA Audit score to 11/11.
The `any` orientation forces the rotation even when the device's screen
rotation is disabled. Using `natural` respects that and restores the
expected behaviour.
The opensearch.xml results in a "site search engine" being added to
Chrome, while the sitelinks search tag results in "Search this website"
being added to Google Search.