new inheritance hierarchy for CronTabber

At the beginning, Socorro and CronTabber shared identical copies of the ...generic_app.App class.  The two projects need to evolve independently.  The socorro.app.generic_app.App class is going away in favor of a new system: socorro.app.socorro_app.App.  Perhaps CronTabber will eventually adopt the new system.  This small change allows CronTabber to continue to work using its own App class as if nothing had changed.  Meanwhile, over in Socorro-land, Socorro is able to take the CronTabberBase class and use it with socorro.app.socorro_app.App.  That makes CronTabber quite flexible.