fix(webapp): fix z layer of header elements

Created by: maeckes-infinitylabs

Changed hardcoded z-index of UI Elements (=overlay, filter post button, floating create button) to SCSS variables.

🍰 Pullrequest

Note: This issue with in the mobile view still exits, but needs a refactor of the filter button and menu and should be put in a seperate issue (see screenshot below). but it isnt critical like the other menu overlapping.

Issues

• fixes #6141

Todo

• None