.form-control and .btn border widths shouldn't be independent?

Created by: cvrebert

.form-control's border is currently controlled by $input-border-width. .btn's border is currently controlled by $border-width. Having these borders be separately customizable can cause complications with .input-groups, since they're all about trying to make those borders overlap, and this allows those borders to be of different widths. I guess it could be made to kinda work by throwing some min()s or max()s into the relevant negative margin expressions. But it might be simpler to just switch .btn over to using $input-border-width. Refs #18417. CC: @mdo