WIP: Redesign GitHub import view

What does this MR do?

Redesign the GitHub import view.

• Split into two columns, docs in the left, action in the right
• Add panel in the right column
• Use non-camelcase text
• Use the settings-message class to add the admin note
• Link to user/admin docs

Inspired from the new protected branches views

https://gitlab.com/gitlab-org/gitlab-ce/tree/master/app/views/projects/protected_branches

Are there points in the code the reviewer needs to double check?

Check that CSS classes are applied correctly. Is settings-message used correctly?

Also, I'm not sure about the button List your GitHub repositories having the same text in both cases for authentication.

The current scenarios are:

1. If the GitHub integration is enabled, you have two choices: either hit the green button directly, or provide a token and hit the button. The current functionality is that if I press either green button even without providing a token, it will perform the same action: take you to GitHub and authenticate.
2. If the GitHub integration is disabled, then we got only the second green button and the user must provide a token otherwise a 500 is shown. I think in this case we should check if the token field is filled by the user and show a warning otherwise.

Why was this MR needed?

To make the import page prettier and add more information for the end user.

Screenshots (if relevant)

GitHub integration (oauth) disabled - before and after

GitHub integration (oauth) enabled - before and after

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Needed for https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5987