swh-web: Add OpenSearch plugin

Summary:
This enables compatible browsers to install a shortcut to the archive search.

Closes T1519

Reviewers: Reviewers, anlambert

Reviewed By: Reviewers, anlambert

Subscribers: anlambert, swh-public-ci

Maniphest Tasks: T1519

Differential Revision: https://forge.softwareheritage.org/D1105