Submenu Visibility & Premium Auto-Updtes fixes

1. Show submenu items like "Add-Ons", "Contact Us" & "Pricing" also when Freemius is turned off (skipped), or pending activation.
2. Fixed the premium plugin version automatic updates mechanism.
3. Following the .org team suggestion, modified the deactivation's form button label from "Deactivate" to "Skip & Deactivate" to make it even more clear that the feedback form is optional.
4. Sticky messages are now hidden from non-admins.
5. Added the option to programmatically start a plugin's trial (only for freemium plugins).

Official release notes: https://freemius.com/blog/may-2016-wordpress-skd-release-notes/