feat: make header sticky

[SukiCZ]

This will add a sticky header

Needs to npm run tailwind:build, I didn't to avoid conflicts.
Fixes: #1019

[lukasvinclav]

This functionality is okay and it can work if you want to hardcode sticky header in your admin but we need more complex solution which will make the header sticky together with the sidebar header as well. Of course, users should be able to opt-in for sticky header from the Unfold config dict.