Fix now ui's navigation mobile overflow menu

[duubyPlz]

Browse how other ui kits do it (they seem to have it okay?):

• get shit done https://www.creative-tim.com/product/get-shit-done-kit
• material kit 2 bootstrap 5 https://www.creative-tim.com/product/material-kit
• https://www.creative-tim.com/product/impact-design-system

[duubyPlz]

May have to open both kits' source codes, then copy css from a downward-dropping kit.

[duubyPlz]

Currently body uses sidebar-collapse class. Once we have the new collapse system, change that (and remove .sidebar-collapse rules @ main.css).

[duubyPlz]

• [x] Copy over css from a downward-dropping kit.
• [x] Make sure the collapse styles look good, colours, content etc.