Jetpack Plugins: hidden sites and their plugins should not be displayed anywhere in Calypso

[v18]

A user can hide a site by going to wp-admin > My Blogs (from any of their sites' wp-admin). Right now, we don't display hidden sites in the sidebar.

However, the plugins for a hidden site do show up on
https://wordpress.com/plugins and its tabs:
https://wordpress.com/plugins/active
https://wordpress.com/plugins/inactive
https://wordpress.com/plugins/updates

Steps to reproduce:

1. Connect a site via Jetpack
2. See that the site shows up on the sidebar on http://wordpress.com
3. Hide the side by first going to the wp-admin area for any of wp.com sites for the same user, and then to Dashboard > My Blogs. You can hide a Jetpack site at the bottom of the page
4. Go to http://wordpress.com/plugins and see that the Jetpack site's plugins are showing up, but the site is not showing up in the sidebar

[v18]

Related: errors about plugins from hidden sites should also not be displayed. For example, if a user has a hidden site and Calypso cannot fetch its plugins (due to a bad connection, etc), they'll see this error message:

[enejb]

Closed via #1489

[jeherve]

Reopening, as the problem still exists:

Most of these sites are hidden from Calypso, but they still generate error messages in Calypso.

Related: #1344