Libravatar Support for Social Media Lookup

Libravatar (https://www.libravatar.org/) is an open-source federated avatar service that can act as a Gravatar replacement for users, and be self-hosted by domain owners, who can publish the location for their server in SRV DNS entries. It gracefully falls back to Gravatar if the e-mail address is not found through their service.
It would be nice to be able to use this as opposed to Gravatar.

The documentation for the API is located at https://wiki.libravatar.org/api/, but it essentially boils down to doing a lookup for the email domain SRV record and replacing www.gravatar.com with the appropriate base directory, using http://cdn.libravatar.org/avatar or https://secdn.libravatar.org/avatar. As an initial implementation for apps already supporting Gravatar, they recommend just doing this replacement without doing the lookup. Since Libravatar falls back to Gravatar (and handles the federated lookup whether using the main site or self hosted), it would be reasonable to simply allow the site admin to set the "Gravatar" address to quickly implement this functionality.

The issue names no files or tests. Read the Libravatar API documentation first, then locate the existing Gravatar address and lookup flow. Done means an administrator can use a Libravatar-compatible address for avatar lookup, including the documented Gravatar fallback behavior.