Add support for tailwind @apply

What does this MR do and why?

When post-processing our CSS with postcss, we can also apply the tailwind plugin in case one of the files contains @apply. We don't need to do that if the files do not contain @apply which will make the process a lot faster.

MR acceptance checklist

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Screenshots or screen recordings

N/A

How to set up and validate locally

1. Use @apply gl-absolute in any SCSS file
2. See position: absolute added in the corresponding css file app/assets/build