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 method of opening the Forum Settings Dialog was edited and added
recursive call for guaranteeing that this Dialog would be 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.

The recursive methods that open and check administration dropdown menu and
main dropdown menu were added because sometimes Selenium just passed
.click() method without any effect or error for some unknown reason.
What dropdown? The name should be less vague.