Update single-sell_media_item.php

Similar changes need to be made to Stock Photography theme page, archive pages etc. I've only just tested it on the single-xxx.php page, but it's a start.
Moving password protection code into central location for sell media icon, #267.

Signed-off-by: Zane M. Kolnik <zanematthew@gmail.com>
Merge branch 'dev' of github.com:graphpaperpress/sell-media into samedgecombe-patch-1