Merge pull request #78 from camilotorresf/pin-loaders

Pin loaders fixes
Add an utility to generate category slugs
Edit the category slug in the admin
Show the category from the slug
Fix category URL in Discover menu

Use the new category URL the Discover menú on the header.
Fix the detection of empty category with sub-category with redirect to the new URL.
Show category links to new URL in pins

In pins that shows the category where they live, put the new category URL.
Also fix the command to delete the tags primary key constraint.
Show new category URL in browse categories
Fix browsing random category

Failed because of the changes introduced by the new URL for categories.
Show sibling categories as related in listing