Use lexical-binding for Gravatar support

Use lexical-binding for Gravatar support

For discussion, see the following thread:
https://lists.gnu.org/archive/html/emacs-devel/2019-07/msg00528.html
* lisp/gnus/gnus-gravatar.el: Use lexical-binding.  Link custom
group 'gnus-gravatar' to 'gravatar'.
(gnus-gravatar-size, gnus-gravatar-too-ugly): Doc fix.
(gnus-gravatar-insert): Check liveness of article buffer sooner.
(gnus-treat-from-gravatar, gnus-treat-mail-gravatar): Use
interactive spec "p" instead of emulating it.
* lisp/image/gravatar.el: Use lexical-binding.
(gravatar-cache-expired): Remove.  Change all callers to use
url-cache-expired instead.
(gravatar-get-data, gravatar-retrieve)
(gravatar-retrieve-synchronously): Simplify.