Improve sign in/out UX

Created by: christinaforney

The sign in page should align with the new design consistency work we've been doing around headers and information architecture. There are some confusing moments with the different sign in options, and the UI could be significantly improved.

Additionally there are a few known issues:

• And the sign in page's nav bar: https://github.com/sourcegraph/sourcegraph/issues/12547
• The sign out page only has a dark theme: https://github.com/sourcegraph/sourcegraph/issues/9810
• Once you sign out, the user flow makes it difficult to sign back in.

Each instance will have different options for signing in depending on what is configured by the Sourcegraph admin. Additionally, Sourcegraph Cloud offers running searches without authentication, but this is not an option for Server deployments.