When safe mode is displayed, and Super User was not logged in already, let...

When safe mode is displayed, and Super User was not logged in already, let Super User display the full safe mode to troubleshoot further (#11082)

* When safe mode is displayed, and Super User was not logged in already, let Super User display the full safe mode to troubleshoot further

* meant for a different PR

* Catch exceptions during CSS/Less compiling

* when there's an error during Twig template processing, or during LESS css compiling, display the safe mode

* Deactivate plugin as super user when authenticated with i_am_super_user

* when user is not logged as Super User, Make deactivate plugin work by forwarding the i_am_super_user URL parameter

* Display plugin version in safe mode fixes https://github.com/piwik/piwik/issues/11043

* Added a text to indicate users to first disable third party plugins

* Updated UI test

* clarify that salt is a secret

* reuse helper method

* minor

* Catching PHP7 errors and making it work on php5