Separated user feed from flyout & activity styles

This should make `user feed` look everywhere the same.
I did that to prevent user feed from breaking if some `nav__submenu` styles get changed. For ex. after last rizzo bump in community autocomplete for users got disfigured because of changes made in `nav_flyout` styles (https://github.com/lonelyplanet/rizzo/commit/4f58666a83fa0bb0ac278fc1aa7e00b7595bf3b4).

Anyways, at least it helps me sleep better! :wink: