Use libravatar instead of gravatar
According to gravatar's privacy policy the data they collect with their service can be used for advertisement. Also the advised caching period with 300 seconds is quite short. All that in combination with the closed source backend makes it probably not the first choice for a free software project. Especially since we have libravatar around, which can be used as a drop in replacement (basically no development cost), does not collect user data and has no advertisement interests according to their privacy policy.
Implementing the simplest from, could be done as in 7d28bdf2 by just replacing the AVATAR_BASE_URI
with https://seccdn.libravatar.org/avatar/
according to https://wiki.libravatar.org/api/.
If wanted, we can then further extend the support for full libravatar support including its federated character over time.