Support for multiple avatar providers, especially libravatar

## Summary

Allow users to use avatars from other services, not just from Gravatar. Especially, support the federated libravatar service which allows organisations to have much more control over how their users' avatars are provisioned.

## Motivation

Avatars are a great way to recognise people as being more than usernames, and although Gravatar is a very long-running service (currently hosted by WordPress) it isn't necessarily good for an organisation to have potentially internal data like email addresses provisioned on a public service. Libravatar provides a mechanism for federation to work behind the scenes, and an org can run their own internal avatar service if they want, or simply allow avatar requests to fall through to Gravatar itself if there is nothing local.
Also, as the gravatar API is simple and well-understood, there are plenty of other pieces of software that can provide avatar data.

Either the user can provide their own preferred avatar service URL, or perhaps the organisation can set one for its users.

## Additional Context

At this stage, this ticket is a request to see what your level of interest is. If you're keen on the feature and want to do it, great! If you would prefer a set of PRs, then I'll look into the code and see if I can figure it out myself :-)