Post-migration experience: Add blog stickers for migration state

[donnapep]

Description

Add blog stickers to indicate the state of a migration as per the following logic. A site should only have one of these stickers at any given time:

• migration-pending – After a plan is purchased in the migration flow.
• migration-pending-diy – When the "I'll do it myself" option is selected.
• migration-pending-difm – When the "Do it for me" option is selected.
• migration-started-diy – When a DIY migration starts.
• migration-started-difm – When a DIFM migration starts.
• migration-completed-diy – When a DIY migration finishes.
• migration-completed-difm – When a DIFM migration finishes.

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[donnapep]

@valterlorran Note that the logic has changed a little to include -diy or -difm for each state. See the issue description above for the details.

[valterlorran]

Hey @donnapep, thanks for the heads up. I will update the logic in my diff. I also want to change from pending-migration to migration-pending to keep consistency.

[valterlorran]

I created a new diff to tackle the updates: D163224-code

[m1r0]

Thanks for the help on this, @valterlorran. FYI, I've just joined the project so don't hesitate to ping me if you need a hand.