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.
1 job for master in 69 minutes and 23 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
failed #5330
test-all

01:09:23

 
Name Stage Failure
failed
test-all Test
make[2]: *** [check-doit] Error 1
Makefile:319: recipe for target 'check-doit' failed
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:294: recipe for target 'check-expensive' failed
make[1]: *** [check-expensive] Error 2
make[1]: Leaving directory '/builds/emacs/emacs/test'
Makefile:959: recipe for target 'check-expensive' failed
make: *** [check-expensive] Error 2
ERROR: Job failed: exit code 1