Previously, calling `sign_in` would cause the browser to be redirected to `/`, and would cause the Ember app to boot. We would then call `visit()`, causing the app to boot for a second time.
This commit adds a `redirect=false` option to the `/session/username/become` route. This avoids the unnecessary boot of the app, and leads to significantly faster system spec run times.
In local testing, this takes the full system-spec suite for chat from ~6min to ~4min.