Extend the genericon class instead of using a mixin
Attachments: Only show other attachments if the attachment has a parent

Before it would show a gallery of other unattached attachments if the
attachment didn’t have a parent.
Add an icon mixin that extends .genericon and adds spacing
Attachments: Hide gallery captions
Attachments: Show a zoom icon before each image size link
Attachments: Only show 'Return to' link if there's a parent
LESS: move some code.

Not sure why this happened.
Breadcrumb improvements

- Use ‘nav’ element as container
- Show the ‘You are here’ text only to screen readers
- Use an icon for the Home
- Use an icon for the separator