Refactor HTML images handling of Gnus and mm-* (a part of bug#21650)

Refactor HTML images handling of Gnus and mm-* (a part of bug#21650)

* doc/misc/emacs-mime.texi (Display Customization):
Remove mm-inline-text-html-with-images; add documentations for
mm-html-inhibit-images and mm-html-blocked-images.

* lisp/gnus/gnus-art.el (gnus-article-show-images):
No need to bind mm-inline-text-html-with-images.
(gnus-bind-safe-url-regexp): Rename to gnus-bind-mm-vars.
(gnus-bind-mm-vars): Rename from gnus-bind-safe-url-regexp;
bind mm-html-inhibit-images and mm-html-blocked-images.
(gnus-mime-view-all-parts, gnus-mime-view-part-internally)
(gnus-mm-display-part, gnus-mime-display-single)
(gnus-mime-display-alternative): Use gnus-bind-mm-vars.

* lisp/gnus/mm-decode.el (mm-inline-text-html-with-images): Remove.
(mm-html-inhibit-images, mm-html-blocked-images): New user options.
(mm-shr): Bind shr-inhibit-images and shr-blocked-images with
mm-html-inhibit-images and mm-html-blocked-images respectively
instead of gnus-inhibit-images and gnus-blocked-images.

* lisp/gnus/mm-view.el (mm-setup-w3m): Use mm-html-inhibit-images
instead of mm-inline-text-html-with-images.