Fix broken left sidebar navigation

Sidebar navigation is broken and it's introduced in !6627. Sidebar pin and hamburger icons were `a` tag before with `#` link on them. That was the main reason why page jumps to top. !6627 was trying to prevent default action but it interferes with the navigation links. I don't want to tweak event listeners for sidebar, instead I just replaced `a` elements with `div` because I think there is no point to have them as `a`.

/cc @dzaporozhets @jschatz1 

PS: This MR basically reverts !6627. I don't want to add another revert commit instead I reverted it in my editor and committed.

See merge request !6648