Add new doctor check for system free space

[acburdine]

Summary

From the forum, we should look at adding a new ghost doctor check to verify that enough free space is remaining on the system before trying to do an install or update.

This can likely use https://systeminformation.io/filesystem.html, specifically the fsSize function. The one bit of logic I'm not sure about yet though is how to determine, from the results of that function, which disk the Ghost installation actually falls under.