Too many alerts/toast messages on a new instance

Created by: nicksnyder

This doesn't create a good first impression for site admins.

There are two separate, but related, issues:

1. Product onboarding (i.e. educating users about literal search)
2. Site admin onboarding (getting Sourcegraph configured)

This issue is focused on (2). (1) is captured in https://github.com/sourcegraph/sourcegraph/issues/6579

Some previous discussion happened in https://github.com/sourcegraph/sourcegraph/issues/6028#issuecomment-549938877