Only an admin or the merchant can edit posts and other DJ Jazzy Jeff stuff

@cgruppioni - please have a look

*Buttons and links only show up if admin, or owner of the post
*Authority is locked down at the controller level
*Product partial updated with seller information, and better flow for readability
*All users can now see categories
*Categories now show the number of items included in them
*Category products now have link to the product
SHould there be punctuation after `Make a new category`?