Stylelint basic setup

[lb-]

• Stylelint is set up and seems to be working, but isn’t running anywhere
• Stylelint config should use https://github.com/wagtail/stylelint-config-wagtail/
• Eslint is not used and should be added
• Eslint config should use https://github.com/wagtail/eslint-config-wagtail
• Stylelint - remove rule overrides #183

[lb-]

Stylelint is set up and now being used #173

Some rules have been disabled as it was going to be a huge change and could not get local built css file to work locally so did not want to change anything functional.

Thanks @PaarthAgarwal for getting this rolling.

[lb-]

split to #183 - closing this issue for now.