Move away from gulp

[artfulrobot]

The project's sass had not been compiled for 2 years, and in that time gulp has changed a lot, and after spending an hour or two trying to get gulp 4 working with the old gulpfile, I gave up.

This PR simplifies the compilation a lot; we now only depend on one npm package, sass, and we run it directly - see the compile-sass-to-css.sh file.

Works for me!

I updated the docs, too.

As part of this work I realised that this extension depends on shoreditch, but only for building its css.

PS. I built this on top of #631 and there was a commit that didn't quite make it into that PR that's included in this. Other than that minor change, I don't think there were any other sass/css changes made.

[mattwire]

Nice, thanks @artfulrobot I wonder if it could be made to depend on https://lab.civicrm.org/extensions/theisland instead of shoreditch?

[artfulrobot]

@mattwire I'm not sure. I'm not sure why it needs to pull in bootstrap stuff - don't we have that in core anyway now? Anyhow, a job for another day!

[MegaphoneJon]

There is no dependency on Shoreditch anymore: #236

[mattwire]

@MegaphoneJon Are you sure? There's no dependency at runtime but seems there still might be for compiling the SASS

[artfulrobot]

Now: build instructions tell you to clone shoreditch inside this ext dir, and this is then referenced by relative paths in:
https://github.com/veda-consulting-company/uk.co.vedaconsulting.mosaico/blob/2.x/sass/mosaico-bootstrap.scss#L1-L16 Can build Sass offline.

Previously: no reference to shoreditch but gulpfile assumed to be able to find it in the normal extensions place. this made the sass build process depend on (a) shoreditch source in the normal ext dir, and (b) being able to run cv in order to discover the dir meaning you had to have PHP and a cv-bootable CRM environment to compile sass.