Auto-select last visited server env on add-on startup
Auto-select last visited server env on add-on startup

- Add initServerEnvironmentPreference helper in addon, which searches
  through browser history for known server environments and finds which
  one was most recently visited to auto-select environment.

Fixes #580

ckprice/testpilot,lmorchard/testpilot,mozilla/idea-town,clouserw/testpilot,6a68/testpilot,mozilla/idea-town,lmorchard/idea-town,meandavejustice/testpilot,fzzzy/testpilot,chuckharmston/testpilot,6a68/testpilot,mozilla/testpilot,ckprice/testpilot,lmorchard/testpilot,6a68/idea-town,6a68/testpilot,lmorchard/idea-town-server,mathjazz/testpilot,mozilla/idea-town-server,chuckharmston/testpilot,lmorchard/idea-town,meandavejustice/testpilot,mozilla/idea-town-server,dannycoates/testpilot,lmorchard/idea-town,mathjazz/testpilot,mozilla/testpilot,ckprice/testpilot,fzzzy/testpilot,dannycoates/testpilot,fzzzy/testpilot,mathjazz/testpilot,mozilla/idea-town,chuckharmston/testpilot,lmorchard/idea-town-server,lmorchard/idea-town-server,flodolo/testpilot,6a68/idea-town,clouserw/testpilot,6a68/idea-town,mozilla/testpilot,flodolo/testpilot,clouserw/testpilot,meandavejustice/testpilot,meandavejustice/testpilot,chuckharmston/testpilot,6a68/idea-town,6a68/testpilot,lmorchard/idea-town,clouserw/testpilot,flodolo/testpilot,fzzzy/testpilot,dannycoates/testpilot,ckprice/testpilot,mozilla/idea-town-server,mathjazz/testpilot,lmorchard/testpilot,dannycoates/testpilot,lmorchard/idea-town-server,mozilla/idea-town,flodolo/testpilot,lmorchard/testpilot,mozilla/idea-town-server,mozilla/testpilot