JOHNSTOWN — For each box unpackaged at the Johnstown School Museum, often found is a missing link in the titular school district’s history.  Within recent months, plenty of boxes have been opened by volunteers. The reorganized museum includes new glass cases and shelves packed with displays dating back as far as colonial times. "What's great is that now if somebody brings a whole bunch of stuff here, we know where we're going to put it," said volunteer and retired teacher Jane Sitterly.  The district was founded by Sir William Johnson near the Fulton County Courthouse in 1764. By the 1790s,…