Fix sidebar not opening first time

Summary:
On devices with <992px width (sm, xs), sidebar doesn't open when swh-push-menu is clicked first time.
This was due to webapp-utils.js:5 setting collapseSidebar to false by default on all devices.

Reviewers: Reviewers, vlorentz, anlambert

Reviewed By: Reviewers, vlorentz, anlambert

Subscribers: anlambert, vlorentz

Differential Revision: https://forge.softwareheritage.org/D1583