Add newcomers onboarding tour

Created by: erzhtor

Closes https://github.com/sourcegraph/sourcegraph/issues/27177. Closes https://github.com/sourcegraph/sourcegraph/issues/29420.

Description

This PR:

• Adds a new onboarding tour for not signed-in users.
• Adds the ability to force override feature flags through initial URL parameters

How to test

1. sg start web-standalone
2. Open the home page and force enable "getting started" tour:
   1. https://sourcegraph.test:3443/search?feature-flag-key=getting-started-tour&feature-flag-value=true
   2. Go through the new tour widget and check it works properly
3. Open the home page and disable the "getting started" tour:
   1. https://sourcegraph.test:3443/search?feature-flag-key=getting-started-tour
   2. Check that onboarding tour doesn't show up and everything works as previously

Screenshots

Before	After
	When tour is OPEN
	When tour is CLOSED