Offer blank topic template if maintainer does not disallow it

Maintainers can set `blank_issues_enabled' to `false' in "config.yml"
to force users to use a template when going through the web-interface.
Respect that here too, but when the value is `true' or unspecified,
then start offering creating a topic from scratch.

Closes ^768.

Author: tarsius

lisp/forge-topic.el

@@ -1900,10 +1900,13 @@ When point is on the answer, then unmark it and mark no other."
                 (buffer-substring-no-properties (point-min) (point-max))
                 :object-type 'alist
                 :sequence-type 'list))
-    (mapcar (lambda (link)
-              `(,@link
-                (prompt ,(let-alist link (concat .name " -- " .about)))))
-            .contact_links)))
+    (nconc
+     (and (not (eq .blank_issues_enabled :false)) ;unset means true
+          '(((prompt . "Blank issue -- Create a new issue from scratch"))))
+     (mapcar (lambda (link)
+               `(,@link
+                 (prompt ,(let-alist link (concat .name " -- " .about)))))
+             .contact_links))))
 
 (cl-defgeneric forge--topic-template-files (repo class)
   "Return a list of topic template files for REPO and a topic of CLASS.")