Multisite testing

[Clorith]

Multisite can be a mysterious beast at times, let's dedicate some time to testing Health Check on it, and finding out what behaviors are the best.

Currently, the plugin can be either network enabled, or individually enabled on singular sites, I feel like this is a good balance, as network admins can then provide the tools to everyone no matter what, or they can leave it as an option for users to take advantage of if they need it.

The capabilities we check for allows both network admins, and site admins to access the plugin, no matter how it was activated.

We should make sure testing doesn't adversely affect anything on the network, and what happens in various scenarios (single site admin with access to more than one site, network admin navigating between sites etc).

[mundschenk-at]

The plugin is not accessible from the network screen, but only on the individual sites. What I've seen it seems to think that there is a problem with plugin updates on all multisite sites, and it lists all globally installed themes and plugins as available on each site, even if they are not.