Form variable definitions appear inconsistent

Created by: Dagothig

Both forms and buttons share variable definitions form the $input-btn- line of variables. However some of the computed default values use the $input-btn- variables rather than the $input- ones. For instance inner height is defined as ($font-size-base * $input-btn-line-height) + ($input-btn-padding-y * 2). Each of these btn variables is also defined so it means that changing the input line height doesn't change the actual input height, but changing the input btn line height does, which comes off as a little surprising.

In a similar manner, the $custom-select variables mix both $input-btn and $input variables when all of the variables are defined by $input.

Overall this leads to situations where overriding all the $input variables only partially changes the values that are used (so for instance the custom select's height does not match the other input heights).