reproducible: add a sane mean to notify the IRC channel when builds finish.

* new column in the database, schedule table
* turn $SAVE_ARTIFACTS a true boolean, 0→no artifacts, 1→yes artifacts
* $NOTIFY in the build script is text containg a keyword:
  + if empty no notification will be sent
  + if "debbindiff" a "$DBDVERSION had trouble with these" will be sent
  + if "failure" a "no artifacts could have been saved"-like will be sent
  + any other value trigs a normal message
* adapt the maintenance script and the manual scheduler to cope with the new
  column

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