Migrate from gravatar to libravatar

This patch replaces all references to gravatar with libravatar, which is hosted by the broader free software community, currently running around Fedora and has no comcerial incentive to invade privacy.

It's also closer to GNOME given that it's just in the neighbour room #libravatar:matrix.org. And it's easy to get a patch in, if there is ever a need for that.

Finally There is no downside to it. libravatar will proxy all gravatar icons anyway. This means there are just more icons for everyone, since now the libravatar crowd is also around, at least those on the central instance.

This patch does not implement the full libravatar API, since there are some tracking related problems with the federation API, when implemented in client applications, that are not trivial to resolve and therefore this is not part of the implementation.

References:
https://www.libravatar.org/
https://git.linux-kernel.at/oliver/ivatar
https://wiki.libravatar.org/api/
#287 (closed)
evolution#793 (closed)