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