Consider rem-calc() instead of calc()

Created by: Kopyov

In some cases, calc() thing breaks CSS: if i set $card-border-width to 0, compiled CSS for card header top border radius will look like calc(0.25rem - 0), which is incorrect. Same happens if $input-height-border variable is set to 0. We either need to document $border-width and other px values used in calc() function properly (value greater than 0 is required) or implement custom rem-calc() function, which will convert all px units to rem and get rid of calc() where possible.