org-latex-preview: Add protection when `untrusted-content' is non-nil

org-latex-preview: Add protection when `untrusted-content' is non-nil

* lisp/org/org.el (org--latex-preview-when-risky): New variable
controlling how to handle LaTeX previews in Org files from untrusted
origin.
(org-latex-preview): Consult `org--latex-preview-when-risky' before
generating previews.

This patch adds a layer of protection when LaTeX preview is requested
for an email attachment, where `untrusted-content' is set to non-nil.

(cherry picked from commit 6f9ea396f49cbe38c2173e0a72ba6af3e03b271c)