Install: save all site IDs at once when network-wide on multisite

[JDGrimes]

Currently we save each one separately, which is sub-optimal.

See #404 (comment)
Related: #703

[JDGrimes]

On further looking into the code, I don't think that we currently save site IDs when installed network-wide at all. add_installed_site_id() is only called in install() when $network is false.

However, this behavior assumes that we will always be installed on every single site on multisite when network installed, which is not true. If we have been inactive a while before we are uninstalled, then we won't have been installed on every site, if new ones were created during that time. See #404 (comment).

I suppose that this also leads to another issue, which is what happens if we were then reactivated rather than uninstalled. I suppose that we wouldn't get installed on those sites that had been newly added, although we should be. So that is another bug that ought to be addressed.

[JDGrimes]

Our install routine does run when the plugin is reactivated though. So perhaps installing on newly added sites would happen automatically anyway?

[JDGrimes]

We decided in #714 (comment) that there really is not a compelling reason to change the behavior to track the site IDs when network-active; as such, this ticket is invalid.

Note that if we decide to revisit this in the future, we'd have to decide what to do there when skipping install on large networks, since in that case we technically wouldn't have installed on all of the sites.