OpenSearch integration for "Add Feed" form

[twm]

As a replacement for registerContentHandler (to be removed in #283), add OpenSearch support to the Add Feed entry box. This will allow it to be added to the Firefox search box, easing copy/paste of URLs into it on a tablet.

See also #12, feed autodiscovery.

[twm]

https://developer.mozilla.org/en-US/docs/Web/OpenSearch