Signup: Single site signup layout bug

[scruffian]

This is only a problem when the user has one site because if the user has more than one site, layoutFocus recognises that the focus will have changed and resets it (https://github.com/Automattic/wp-calypso/blob/8b9be6225433a058c07a4ae9b424cf29fe828a29/client/lib/layout-focus/index.js#L68). When the user has only one site the layoutFocus has never been set, so the router doesn't know to reset it. Another solution would be to just always reset the navigation to content on navigation, but I guess this will be annoying for mobile users who want to keep the sidebar open.

Fixes #442

Testing

1. git checkout fix/442-signup-layout
2. Open http://calypso.dev:3000/start in an incognito window
3. Create a site
4. Click on "My Site" (you need to only have one)
5. Click on "Add New WordPress"
6. Assert that the signup loads correctly.

• Code review
• QA review

cc @designsimply

[gziolo]

This is not related, but adding it here. I was trying to add site for the user that had blog marked as spam:

[gziolo]

QA passed 👍

[gziolo]

Code looks good 👍

[artpi]

Works, the only issue is:
after creating SECOND site, on the screen with "Customize / etc" - the one just after creation, the top bar stil is "My Site" instead of "my Sites"
https://cloudup.com/c49ibiD8d9Y

But after clicking skip it changes to "My Sites"

[gziolo]

@artpi: nice catch, though this is also not related to this change, we need to create follow up PRs.

[scruffian]

This is not related, but adding it here. I was trying to add site for the user that had blog marked as spam

Reported in #471

[scruffian]

Works, the only issue is: after creating SECOND site, on the screen with "Customize / etc" - the one just after creation, the top bar stil is "My Site" instead of "my Sites"

Reported in #473