#7878280 Staging Sites: Customize progress message depending on the user

See Automattic/dotcom-forge#1857

Proposed Changes

Uses a different progress message depending on whether or not the user is the site owner.

Non-site owner

Site owner

Testing Instructions

1. Create a new Business site and take it Atomic.
2. Apply D105036-code to your sandbox if it isn’t yet committed.
3. Navigate to Hosting Configuration.
4. Click ‘Add staging site’ and verify the message appears as expected to the site owner.
5. Delete the staging site.
6. Add another user as an administrator to the site.
7. Navigate to Hosting Configuration under that user.
8. Click ‘Add staging site’ and verify the message appears as expected to the non-site owner.