Issue 6788 - Integrated stylelint-config-eyeo

This MR for #79 (closed) contains the following changes:

• Added lint.css command to package.json.
• Added .stylelintrc:
  • Disabled "block-closing-brace-space-after" rule (see trac#6789).
  • Temporarily disabled "selector-no-qualifying-type" rule since those require manual changes and there are quite a few of them and therefore might block the initial integration of CSS linting.
  • Updated "selector-type-no-unknown" to allow selectors to refer to our custom elements. If we agree on using this, we can also ask this to be added to stylelint-config-eyeo since it's likely not just useful for us.
• Ran stylelint --fix to automatically fix violations (see https://gitlab.com/eyeo/adblockplus/adblockplusui/commit/790b86ee1e6b044fce4612ea9cbd84aa8b289b67).
• Manually fixed remaining violations (see https://gitlab.com/eyeo/adblockplus/adblockplusui/commit/b497e44de25c6d1ee7c846e871c64ef27b5fb7e0).

Note the rule conflicts mentioned in #79 (closed) as well as the following limitations with the /* stylelint-disable */ feature:

• stylelint#2643: Ignored when running stylelint --fix
• stylelint#3111: Ignored when put within select list