Platforms - two menus displayed on screen.

[calumcraig]

Steps to reproduce:

1. Launch hub.safedev.org, go to platforms page.
2. Resize browser window (smaller) so that navbar collapses to hamburger menu and sidemenu also collapses to a hamburger menu in bottom RHS.
3. Click on both hamburger icons.

Expected:
TBC
Actual:
Both menus are displayed, one top of another: