Removed unused legacy code
new method: bucket3.util_slugify()

Used to generate post slug if not manually set.
Introducing log/ dir

Messages, logs and other "output" should go in the log/ dir.
Not much yet, only external_links.txt has been moved there.
Fix: mentions_get() was removed by mistake.

Added bucket3.mentions_get() that was removed by mistake
in previous commit.
Typo fix.