Custom select validation tweaks

[MartijnCuppens]

The padding of custom selects doesn't cover the background, if the text in the select box is to long, the text and background image will overlap.

This PR also removes unneeded calc() functions.

Before:

After:

[mdo]

I wonder if we take the opportunity to clean this up a bit here and turn these major values into variables? Separately, was the / working as intended before? Curious if Sass was trying to compute this vs keeping it as a property separator.

[MartijnCuppens]

A calc() function was used, that's why the / worked before.

Do you want to use the variables in the _variables.scss file or declare them here in the _forms.scss?

[mdo]

Ah, yeah, got it. Most likely in _variables.scss.

[XhmikosR]

@MartijnCuppens: can you move the variable to _variables.scss?

[mdo]

Tests are failing for linter issues.

[MartijnCuppens]

I've moved some values to the variables file as mdo suggested and simplified the if/else construction. Also fixed the linting issue and squashed everything into one commit.