Add support for multiple Gravatar services

Add support for multiple Gravatar services

Now supports Libravatar and Unicornify, next to Gravatar (Bug#39965).

* lisp/image/gravatar.el (gravatar-base-url): Remove constant.
(gravatar-service-alist): List supported services.
(gravatar-service): Add user option to specify service, defaults to
Libravatar.
(gravatar--service-libravatar): New function, libravatar image host
resolver implementation.
(gravatar-build-url): Use alist gravatar-service-alist instead of
gravatar-base-url.
* etc/NEWS: Mention new gravatar service option.