Prefer HTTPS to HTTP for gnu.org

Prefer HTTPS to HTTP for gnu.org

This patch just changes code files; a followup companion patch
(much larger) will affect the commentary.  This part is
separated out to make it easier to review.
* .dir-locals.el (change-log-mode):
* lisp/org/org-info.el (org-info-other-documents)
(org-info-map-html-url):
* lisp/org/ox-html.el (org-html-creator-string):
* lisp/startup.el (fancy-startup-text, fancy-about-text)
(fancy-splash-head):
* test/lisp/ffap-tests.el (ffap-other-window--bug-25352):
* test/lisp/thingatpt-tests.el (thing-at-point-test-data):
Use HTTPS instead of HTTP.