Add a floating subscribe button, which, when pressed, opens the subscribe modal:

We now can render "button only" subscribe block variation, and the email will be asked in a modal.

The floating button would be enabled via "Jetpack -> Settings -> Newsletter" alongside popups and other placements:

The button should be pulled from a template part, so that we can let folks jump in and modify the button's appearance, add some copy around it if they wish, etc.

Currently, this would clash with the "action bar" on WP.com sites:

We should keep showing the button even when subscribed when site has paid subscriptions; the button should then nudge to upgrade.