Improve self-hosted site login for Jetpack & WP

Actual Behavior

Self-hosted login allows you to go through the whole login flow with a site you’re already logged into

Expected Behavior

If you are already logged in to the site, the login flow would terminate early and show a message.

Steps to reproduce the behavior

• Connect a self-hosted site to a WP.com account
• Login to WP or JP with the same WP.com account that you just attached your self-hosted to
• Navigate to My Site
• Tap the drop down caret next to the Site Name to launch the Choose Site view
• Tap the more menu in the upper right hand side
• Tap "Add new site"
• Tap "Add self-hosted site" in the pop-up modal
• Follow steps to log in to the self-hosted site
• Note: At the end of the flow, it gives the error “Couldn’t select newly added self-hosted site”. This displays over a quick-start prompt for an existing site if logged into WordPress.com.

Improvements

• Login flow should detect earlier that we’re already logged into this site.
• Quick-start shouldn’t be triggered if the login flow can’t complete.

Start by reproducing the self-hosted login flow described in the issue and trace where the existing-site check and quick-start prompt are handled. Done means the flow ends early with a message for an already logged-in site and does not trigger quick-start when site selection cannot complete.