Tests stability improved

Fixed logical error when some methods in 'Existence' class changed
web-driver's implicit wait for lower timeout between actions but didn't
change it back to normal.

The methods of opening the Forum Settings Dialog and External Links Editor
were edited in quite similar way for guaranteeing that these dialogs
more likely would be successfully opened.

The method of switching admin mode was edited and separated according
to the wide- or small- headers.

Also isAdminModeOn() check was significantly accelerated. Previously it
used standard webdriver timeout that wasn't necessary absolutely.

Unnecessary @Override annotations were removed in wide-/smallScreenHeader
classes to improve the code visually.

Methods that open and check administration dropdown menu and
main dropdown menu were edited to be more stable because sometimes
Selenium just passed .click() method without any effect or error for
some unknown reason. Two consecutive checks would be made before exception
will be thrown.