Use vsite_register's context-aware "Create a site" block, not new box #4663

#### Changes

* Modify header block text
* Move vsite_register's block into the correct region
* Add variables to configure "Create a site" block text
* Add contextual link to block display for sitewide admins
* Theme link nicely

##### Todo

* Use `drupal_get_path()` for image src attributes in `os_front.box.inc`