Overhaul sign-in page for self-managed GitLab

Currently the default sign-in page on GitLab has stale content. We should update it to include our latest marketing copy. The UI for signing in and registering also needs attention, per @sunjungp's comment below.

Current default sign-in page:

---

Proposal

Below is the proposed language to use:

Title

GitLab

Body

### A complete DevOps platform

GitLab is a single application that is fundamentally changing the way Development, Security, and Ops teams collaborate. With GitLab, you get a complete CI/CD toolchain in a single application. One interface. One conversation. One permission model.  Get started today.

Sample sign-in page