#2750699 Site migration: redirect to homepage when migration is complete.

Changes proposed in this Pull Request

• When a migration is complete we reset it and redirect to the Customer Home.
• If a site has been the target of a successful migration in the last two days, we show a custom header on the Customer Home.

The design also calls for a customised checklist on the Home – we’ll look at that in a separate PR.

Testing instructions

• Apply the PR and run Calypso locally, or use calypso.live (for the latter, add ?flags=tools/migrate to the end of the URL to enable the feature flag).
• Do a migration: go to the import section of a Simple site, enter the URL of a Jetpack site and follow the steps you’re presented with. (If you’re migrating to a site you’ve migrated to whose done status hasn’t been cleared yet, you may need to reset the migration first with a request to the reset-migration endpoint.)
• When the migration finishes you should be redirected to the My Home, unless there was an error.
• If you visit My Home for any site that has been migrated in the last two days, you should see the special header on the page telling you that your site has been migrated. For any other site, you should see the default header.