Closes#6440
* Removed the `.fake-placeholder`class from the input, test, and css
* On adding a nav item, if the url value has not been set by the user,
then set it to the base url as shown in the input
* If url has been set by the user, just do what it has always done
refs #6458, closes#6457
- replaces jquery-ui.sortable with ember-sortable for drag-n-drop handling
- moves the "new/blank" nav item out of the nav items list
- allows it to be excluded from the draggable list
- cleans up handling of the `navigationItems` array as there's no longer a need to ignore/exclude this extra item
- clears validation errors when typing in the respective field
- adds acceptance test for adding/removing nav items
- improves acceptance test for saving nav items to cover more edge cases
issue #5841
- fix relative link checks in navlink url input component
- fix navlink url input component sending absolute URLs instead of relative URLs to action handler
- remove URL manipulation in navigation settings controller (url input handles URL manipulation, validator flags anything that's still incorrect)
- capture cmd-s in url input to ensure changes are actioned before save
- automatically add mailto: to e-mail addresses
- add gh-validation-state-container component so .error/.success validation classes can be applied to any container element
- add validation-state mixin that can be mixed in to any other component to give it access to validation status (used in gh-navitem component to keep alignment when inline error message elements are added)
- validate and display inline errors on save
- improve ember test coverage for navigation settings related controller and components
