[do not merge] trying out sass with gov uk elements

[vygis]

I spent a day looking into switching Bootstrap Lass to use Sass, and include at least some of the styles from Gov UK elements directly (Gov UK uses Sass).

I've come to the following conclusions:

• it would be feasible to replace the less based Bootstrap 3 module with an official Boostrap 3 Sass port. However, it would not be straightforward because sass and less have different functions (especially around calculating height and em values), and some of the existing &:extend(.class all) less statements cannot be mapped directly.
• it is not feasible to consume all of the Gov UK style resources. The header and footer styles are only provided by the govuk_template npm package in their compiled form, together with a server side template, so it would not make sense to even attempt to integrate them. Some of the other styles that are provided by govuk_elements would be more realistic to integrate, but it would need a complex set of includables and variable overriding, especially when combining with Bootstrap. Even then, govuk_elements do not provide everything we need, so they would need to be extended, which begs the question of including them in the first place.

I would therefore recommend not to go down this path and continue using Bootstrap, unless there is a pressing need to use Gov Uk styles.

Having said that, a lot of the existing Bootstrap customisation in ngGovUk could be improved and made to fit Boostrap better. I would suggest focusing on that.