Components: Adds LoggedOutForm component

[ebinnion]

In an effort to factor the logged out link that shows at the bottom of the form at /start/account/user and to allow reuse of the logged out form styles, this PR refactors the signup/logged-out-form component and brings much of its functionality to client/components/logged-out-form-container component.

cc @mtias and @rickybanister for design review, @lezama, and @drewblaisdell for code review.

To test:

• Checkout add/components-logged-out-form branch
• Test invite flows
  • On a WP.com site, invite a test user by going to $site/wp-admin/users.php?page=wpcom-invite-users
  • In the invite email, make note of the invitation key
  • Go to /accept-invite/$site/$invite_key
  • While logged in or out, can you accept an invite? Are there errors? Does everything look OK?
• Test user creation
  • Go to /start/account/user
  • Attempt to create a user
  • Does everything work and look good?
• Test site creation
  • Go to /start/site-user/site
  • Does everything look ok? Are there any errors?

Note: You'll notice that the /log-in form is off centered, and during testing you will likely run into a JS error if you try to submit the form on /log-in. This is not related to this PR and /log-in is only enabled in development.

Also note: In some screenshots you'll notice there's quite a bit of padding between the form footer and the input directly above it. This seems to be caused by:

.validation-fieldset + .logged-out-form__footer {
    margin-top: 20px;
}

I intentionally added a screenshot with an invalid form field so that you could see what that would look like.

After screenshots:

[enejb]

missing propTypes: {} The same is true for the rest of the new components.

[enejb]

I left some minor comments. Overall in my testing I didn't find any errors beside the once that you pointed out already. I was able to accept the invitation + create a new user and a new site.

[ebinnion]

@enejb I went ahead and ES6-ified and added propTypes to address your comments.

[ebinnion]

Added missing border-top to the form footer.

[rickybanister]

What's the email-address-only 'log me in' form for?

The footer looks good in all the examples. This seems like a useful refactor.

[ebinnion]

@rickybanister I'm not entirely sure. I think it was an attempt at mobile login, but it hasn't launched yet.

The idea seems to be that you enter your email, and then you'll receive a one time pin which you can enter in the next step. But, the form was broken against master last time I tested.

[lezama]

Tested and everything seems to work including /start 👍
This looks much cleaner and ready to go to me :)