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