Ignore generated zip files.
Move executable SQL upgrade snippet below cache change stuff
and add changes to special_{run,compare} and compile_script columns.
Calculate timestamps of example contest data at insert time.

This ensures they are in the same idea of timezone that the host system has,
rather than the timezone in which the DOMjudge developers live. This is
necessary to make them correspond to the human-readable _string versions
in the same table.
Fix SQL upgrade snippet for executable scripts, now tested ;-)