[web] Add a temporary login via QR code button on the log-in form

Summary:
While the QR code work is still in progress, we want to display a login via QR code button hidden by an if (isDev) check to ensure it's not shown in production (also because ultimately the screen's location will be moved to the main login method, but while it's impossible to login via QR at the moment, this is the next best way to access the flow).

Addresses ENG-4584

Confirmed with Ted regarding the designs of this button in the design channel on Comm

Depends on D8806

Test Plan:
Confirmed that the button shows in development

Reviewers: atul, ginsu, tomek, varun

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D8811