Sidebar submenu needs attention

[aero31aero]

A lot of stuff about the sidebar needs fixing, including:

Subtasks

• [x] Sliding in and out on small displays.
• [x] The sub menu is not centered but main menu is.
• [ ] The sub menu architecture. Make it appear in between the main menu.
• [x] Other random fixes, if any.

Assigning 1 and 3 to @rogeredthat because he's the only one capable of doing that as of now.

[rogeredthat]

Is this over?

[aero31aero]

See the third point. I'll do a rough JS version PR and leave the CSS to you, I guess.

If its a WONTFIX then we'll just close it.