Add OpenSearch registration

[plaidfinch]

OpenSearch allows a site to register itself as a search engine that can be installed in the user's browser. Adding this metadata to the RFD site would allow users to quickly install it as a shortcut. I can see two ways this could be done:

1. Register https://rfd.oxide.computer/%s so that "searching" is just looking up a numerical RFD. This is very easy.
2. Register an endpoint which redirects to a numbered RFD if the search is a literal number, or engages full-text search if not (I suspect this endpoint would need to be created).

This work could be phased: (2) is a generalization of (1). However, if some users install the search engine after (1), they would not receive the benefits of (2) unless they manually re-register the search engine again. For this reason I suggest simply implementing (2) directly.