Use imports everywhere

[jgthms]

This is an improvement.

Proposed solution

Instead of relying on the ancestor Sass file to import dependencies, each component file imports its own dependencies.
Fixes #2797

Tradeoffs

No real tradeoff, unless some old Sass version does not support importing the same file multiple times.

Testing Done

Tested with latest versions of:

• Dart Sass npm sass@1.29.0
• Node Sass (LibSass) npm node-sass@5.0.0
• even Ruby Sass sass-3.7.4.gem

Changelog updated?

Yes.

[lionzeye]

There is a lot of duplicate css in the output file after this was merged. Selector
.image.is-1by3 .has-ratio, .modal, .modal-background, .is-overlay, .hero-video
appears in the output file 40 times now. See the output in https://github.com/jgthms/bulma/blob/38aa8b9299cb70f165783858122bcc7f6e3d6d35/docs/css/bulma-docs.css for example

[jgthms]

You're right, I've noticed that too. It's how @extend rules are populated. I thought cssnano might catch it and optimize it, but it didn't. I will create a new extends.sass file in the /utilities folder which will only be imported once.