Remove the usage of the "inputbox" class from backend

Currently, we use the CSS class `inputbox` in 391 places in the backend, however it is only a single CSS rule dealing with it. And this one can be removed without any issues.

This PR removes the class from the backend, but leaves frontend (and libraries) untouched.

The one CSS rule which deals with that class is found in Hathor and targets the login page. For LTR languages, there is no change at all when we remove the rule. For RTL languages it even looks better :smile: 

There are two goals with this PR. First it removes unused and inconsistent output and second it will make it easier to deal with "chosen" as we can use a default class for `select` elements.

I targeted this PR against 3.3 because there is a theoretical chance of breaking 3rd party administrator templates if they exists and work with this class.