Jetpack Connection: implement connection screen as an actual modal

In the Jetpack connection flow, when users click _Finish setting up Jetpack_ on the _My Jetpack_ page, they're brought to a different page that implements a modal embedded in the page. Let's implement it as a proper modal in the _My Jetpack_ page.

More context in this post: p6TEKc-7DI-p2

Start at the My Jetpack page and follow the Finish setting up Jetpack action through the Jetpack connection flow. Compare the current embedded modal page with the screenshots and the linked p6TEKc-7DI-p2 context; done means the connection screen opens as a proper modal on My Jetpack.

Written by the indexing model from the issue text.