TapQueue server and station 0.17.0

Added

• Released jobs are followed at the printer. "Released" used to mean the printer accepted the
  job; what happened to it afterwards was never looked at, and the document was already gone. Now
  the server asks the printer about each job until it's finished and keeps the document that
  long. A job the printer aborts is held again, counts against no page limit, and can be released
  at another printer. A job stopped at the printer's panel gives back the pages that didn't print
  when the printer says how many did. The console, tapqueue-admin jobs and the API's printState
  say printing, printed or stopped, and printedPages is the printer's own count
  where it gives one. Printers that can't be asked about a job behave as before. Database schema 19.
• Queues offer what the printers can do. A queue used to tell every PC the same thing: four
  paper sizes, 600 dpi, one tray, no finishing, whatever the printers behind it were. It now
  offers the paper sizes, trays, paper types, finishing and resolutions the printers report, the
  document formats every printer takes, and colour and two-sided only if some printer can do them
  (and the queue's settings allow). A job that asks for a paper size is released only at a printer
  that has it; elsewhere the release is refused with the reason and the job stays held. A colour or
  two-sided job released at a printer that can't prints in black and white or on one side, the
  release result says so (note), and the job is recorded as it came out. See
  docs/how-it-works.md.
• A screen at the station. A tap that found nothing waiting, one that was refused (over a
  page limit, a printer that can't print, not allowed there) and a card nobody has all looked the
  same at the printer: nothing happened. Each station now has a screen page that any browser
  next to the reader can show, on a tablet, an old phone or a monitor: green with a tick for
  printing, amber for nothing waiting, red with the reason for a refusal or an unknown card, and
  between taps whether the printer is ready. Screen link on a station in the console, or
  tapqueue-admin stations screen <id>, makes its address. The station program isn't involved,
  so it works with stations on any version. Database schema 20. See
  docs/release-station.md.
• A tap that printed nothing is explained on the person's PC. With a 0.17 client, the tray
  says "You tapped at Office printer, but nothing was waiting to print", or why a release was
  refused, or that a printer gave up on a job and it's waiting again (GET /api/v1/me/notices).
• Downgrade. The Server page has a Downgrade section (tapqueue-admin server downgrade) that
  takes the whole site back to an older release when an update turned out badly: it hands out that
  release's clients and station build, then installs that release of the server. The running
  server steps its database back first, after making a backup. 0.17.0 is the oldest release that
  can be gone back to. See docs/install-server.md.

Changed

• Client and station builds only come from GitHub releases. Uploading a build is gone: the
  upload forms on the Updates page, tapqueue-admin clients publish, and POST to
  /api/v1/admin/client-builds and /station-builds. Nobody with access to the console can hand
  PCs and stations a program of their own any more. Use Check for updates (or
  tapqueue-admin clients update and stations update), which needs the server to reach GitHub.
• Stations check an update against GitHub before installing it. A station used to install
  whatever build its server handed out. It now fetches the release's list of programs
  (SHA256SUMS.programs) from GitHub itself and installs the build only if it's on it, so a
  server that was taken over can't run a program of its own on the stations. Stations need to
  reach github.com to update. releases_url in station.toml is for sites that build their own.
• The clients share this version number from now on (see the clients' changelog).