Make gravatar.el more configurable

Make gravatar.el more configurable

For discussion, see the following thread:
https://lists.gnu.org/archive/html/emacs-devel/2019-07/msg00528.html

* etc/NEWS: Announce changes in gravatar.el user options.

* lisp/image/gravatar.el (gravatar-cache-ttl): Change :type to
number of seconds without changing the default value and while still
accepting other timestamp formats.
(gravatar-rating): Restrict :type to ratings recognized by Gravatar.
(gravatar-size): Allow nil as a value, in which case Gravatar's
default size is used.
(gravatar-default-image, gravatar-force-default): New user options
controlling the Gravatar query parameters 'default' and
'forcedefault', respectively.
(gravatar-base-url): Use HTTPS.
(gravatar--query-string): New helper function to facilitate testing.
(gravatar-build-url): Use it.

* test/lisp/image/gravatar-tests.el (gravatar-size)
(gravatar-default-image, gravatar-force-default)
(gravatar-build-url): New tests.