Wp-calypso: Add gauge on My Home link about checklist completion

[davemart-in]

Description

We've seen from usability studies and concierge calls that it can be confusing for some people when first getting started where to go to get back to their launch checklist.

We'd like to add a visual indicator on the My Home link which both hints at where the checklist lives, and shows you an indication of how far along you are.

Here's a mockup:

[Aurorum]

Started a PR for this in #42022. :)