Plugins: Always display Plugins section for WordPress.com Business sites

When my WordPress.com site on the Business plan is set to private, the Plugins section doesn’t show up at all under My Sites.

That is confusing — I have no indication what I needed to do to make the section appear (and as a regular user I might just assume that I can’t do anything related to plugins in the app). It would be great to have some indication for the user, at least when the site has a Business plan, that explains what they need to do to use plugins on their site. Here’s how Calypso does it (after clicking the Install button on a plugin):

The issue is compounded on the iPad, where the site menu is visible at all times and not refreshed after making changes to things like site settings. That causes two problems:

1. If the site is private, making it public doesn’t make Plugins appear until I leave the screen (e.g. go to another tab like Reader or go back to the site picker) and return to that site.
2. If the site is public, making it private doesn’t make the Plugins section disappear. I can then go to Plugins, try to install a plugin, and get the register domain flow even though my site doesn’t qualify for automated transfer.

The second scenario especially points to a need for some kind of check before starting the automated transfer flow that doesn’t depend on the Plugins section being visible or hidden.

Steps to reproduce the behavior

1. Set up a WordPress.com site with the Business plan.
2. Set the site to private.
3. In the app, go to My Sites and look for the Plugins section for that site. It doesn't appear and there's no indication why.

Tested on iPhone XS & iPad Mini, iOS 12.1, WPiOS 11.3.0.20181119

Start by reproducing the private and public Business-plan site cases on iPhone and iPad, focusing on the My Sites Plugins section and the automated transfer flow. The issue names no files or tests, so trace the menu refresh and plugin-install entry points from the observed behavior. Done means the Plugins section and transfer flow remain consistent with the site's visibility and plan state.