Custom checkbox alignment possible regression

Created by: sbaechler

The checkbox alignment was changed in this commit: https://github.com/twbs/bootstrap/commit/82e7abcdf301f27096ee180a53a08a1107e8da1a.

This broke the alignment on our page. I have not overridden the vertical alignment of the element.

That magic number on line 64 looks a bit suspicious to me. Maybe the top margin needs to be calculated.

My $custom-control-indicator-size is 1.5em. Maybe the top value can be calculated from that value and the line height.