Include update site info in statbox

[imagejan]

We should make it as easy as possible for users to see how to install a given plugin. When a plugin is available via update site, let's highlight the update site name and/or URL in the statbox (together with a link to the general documentation on update sites). For plugins without update site, we can fall back to "Installation: manual" or "Installation: custom".

See discussion on Gitter.