Show destinations in the list of schedule links

Rather than them being hidden under a mysterious "destinations" link.

Also removed the `$is_tab_visible` property and replaces it with a `has_form()` method on the parent class.

Introduces a `get_slug` method on the parent class which returns a sluggified version of the destination name.
Only show excludes for backups which include files
Remove uneeded $is_tab_visible
Move the error check outside of the foreach
Codeing standards
Merge branch 're-design' of github.com:humanmade/backupwordpress into re-design