Site creation rework: Offer a "Close" button on failure after creation

[hypest]

Fixes #7221

Shows a "Close" button when the site-creation process finishes but the new site has been created remotely. If it hasn't, a "Back" button is displayed.

To test #1:

• With the app logged in to wpcom, go to the site picker and start creating a new site. Bring the wizard up to step 4 (subdomain selection) but wait.
• Turn connectivity off
• Push the "CREATE SITE" button
• The "Creating" screen comes up, it informs that something went wrong and it displays a "Back" button on the toolbar
• Tap the "Back" button and notice the wizard returning to step 4

To test #2:

• With the app logged in to wpcom, go to the site picker and start creating a new site. Bring the wizard up to step 4 (subdomain selection).
• Be ready to turn connectivity off after the "Creating" screen comes up
• Push the "CREATE SITE" button and wait for the process to reach the "Retrieving site information" step
• Turn connectivity off
• The app informs that something went wrong and it displays a "X" button on the toolbar
• Tap the "X" button and notice the app returning directly to the site picker

[theck13]

Everything looks good, but there are a few conflicts to resolve before merging.

[hypest]

Closing this PR in favor of #7247 as it turns out the commit included here already got reviewed in #7247. Will copy the testplans there as well.