The width of the straps was calculated by multiplying with the `sa`
value.
As a result, when the user asks to draft a pattern without SA, the
straps drop off the pattern.
I've also change the way the width is calculated. It used to be based
on a percentage option that was then multiplied by SA.
While technically that follows the best practice of using percentages,
it won't scale with the model unless you multiply it with a measurement.
- This measurement is not listed as a required measurement in the
config, thereby making this pattern throw an error in the frontend
- Furthermore, the `hpsToWaistFront` measurement is (currently)
listed as a breasts-only measurement. Making it a required
measurement would make this pattern a de-facto womenswear pattern.
As that seems a bit weird for an apron, and the measurement was
only used to construct the pocket, I simply swapped it for
`hpsToWaistBack` (which was already a required measurement)
