[Major upgrade]: Upgrade to Tailwind v2

[pixelastic]

Norska should update to Tailwind v2. I assume some of my custom plugins will be integrated into the core. Thankfully, we now have Percy tests to make sure we didn't break anything, so the upgrade should go smoothly.

• Handle the postcss-nested situation. Either pin it, wait for a fix, or stop using it.
• Update Tailwind to v2
• Remove custom plugins that overlap default feature of v2

[pixelastic]

Upgrade of postcss-nested is currently blocked as it creates node.getIterator is not a function errors (even when not calling it).

See postcss/postcss-nested#106 for details on the error.

[pixelastic]

postcss-nested wasn't the issue. The issue was postcss-clean, and we're now using cssnano. So the whole postCSS situation is solved.

• Tailwind text-color classes should work when defining custom colors #125 We should allow overriding default colors from the host