Update Login Screen UI

### Why
In support of #2557, this issue aims to provide an initial task to help stakeholders get acclimated to the project.

### Goal
Implement an updated login screen UI based on mockups by @AustinCondiff:

https://projects.invisionapp.com/share/3ATIRBHKW9F#/screens/380522637

### Details
- The mockup shows the Git Gateway password login screen - that UI comes from the [netlify-identity-widget repo](https://github.com/netlify/netlify-identity-widget/) and is non-trivial to customize. Recommend considering that part out of scope for this issue.
- Due to the previous point, a mockup should be made showing how the other backends, which currently consist only of a button instead of a form, should appear.
- The current login components use some strange positioning, which led to https://github.com/netlify/netlify-cms/issues/2133 being opened. That issue can probably be resolved under this one.

### Tasks
- [ ] Create additional login screen mockup with oauth button in place of form - @AustinCondiff
- [ ] Implement design and document related components in Storybook - @danoszz & @timmysmalls

### Prerequisites
- #3102

Review the login components, the linked mockup, prerequisite #3102, and related issue #2133 first. Confirm the additional OAuth-button mockup before implementing the updated login design, and document the related components in Storybook; completion requires the agreed design and Storybook coverage.

Written by the indexing model from the issue text.