Fix showing build steps in the machine status page

This was broken because of the comparison

  step.machine.match('@(.*)').0 == m.key

where m.key contains the SSH user.

Also some style tweaks.
Machine status: Don't show removed machines anymore

This requires a sequential scan on the BuildSteps table, which by now
takes > 8s.
Remove some redundant SQL queries
Make negative narinfo responses much smaller
Minor tweak