machine-status: Read /etc/nix.machines instead of using the BuildMachines table
machines: Read /etc/nix.machines instead of using the BuildMachines table
Remove all entry points to modify machines
Remove the BuildMachines and BuildMachinesSystemTypes tables
hydra-queue-runner: Use nix.machines instead of the SystemTypes table to determine how many build jobs are allowed per system type.

Note that on machines that support multiple system types, EACH system type gets the full number of build slots, which is almost certainly not what we want.
Remove now-unused SystemTypes table
Show inputs and input changes on the jobset eval page
Add a button to compare a jobset eval against other jobsets

So now "?compare=<jobset name>" is no longer a hidden feature!

P.S. Encountered this wonderful TemplateToolkit brainfuck again: if
you want to get the number of rows in (say) project.jobsets, you can't
say "project.jobsets.size".  That will *usually* give the right
result, except that if there is only one row in project.jobsets, it
will evaluate to 3.  Instead you have to use "project.jobsets_rs.count".
Revert "Remove now-unused SystemTypes table"

This reverts commit 71d020735bb6f8e2f46a7813fe73d2a8dbe37add.
Unfortunately there are still some cases where we need to set Hydra's
concurrency separately.  (Ideally, Hydra would start *all* queued
builds in parallel and let Nix take care of everything...)
Respect SystemTypes if defined