Add `!important` to `hide` and `show` utilities

Created by: necolas

I'd argue that these utilities should always hide and show. This would also avoid you needing to redeclare them in certain parts of Bootstrap where the specificity of a rule means their display value is not respected.

One thing to note is that this would affect any JS that tries to hide/show elements using inline styles rather than applying/removing these classes to/from the HTML element.