[infrastructure] Add (S)CSS linting infrastructure

[traviskaufman]

• Add Stylelint config
• Add BEM linting
• Add SCSS linting
• Fix all general lint errors
• Remove theme scss files as we won't be using them moving forward

Part of #4464
Relates to #4539

NOTE: Assigning everyone since this should get feedback from all core team members

[Garbee]

We should actually enforce using HTTPS wherever possible, not relative URLs.

[traviskaufman]

I think that's what setting this to true does https://github.com/stylelint/stylelint/blob/master/src/rules/function-url-no-scheme-relative/README.md

[Garbee]

AH, OK. I was searching around and saw the initial issue. They were saying relative URLs there were good and should be enforced, so I thought this was doing that instead.

[Garbee]

LGTM

[rfru]

LGTM
Very nice!

What are the // postcss-bem-linter: comments used for? (besides ignoring)

[traviskaufman]

@rfru it tells postcss-bem-linter that the code contained in between them is a BEM component - https://github.com/postcss/postcss-bem-linter#defining-a-component

I put the end comments in to avoid potential issues with sass concatenating files together and having the linter throw false positives.

[sgomes]

We've been doing double quotes in CSS and single quotes in JS, but I suppose single quotes everywhere works too.

[traviskaufman]

Ah okay no problem. I can change this. I don't mind dbl-quotes anyway. They look like attributes and that's what's used in the WebIDL for attrs (AFAIK).

[sgomes]

LGTM, can't believe the range of options available!