When generating models, we created ApplicationRecord in the default
location if no file existed there.  That was annoying for people who
moved it to somewhere else in the autoload path.  At this point, the
vast majority of apps should have either run the upgrade script or
generated a model since upgrading.  For those that haven't the error
message after generating a new model should be helpful:

   NameError: uninitialized constant ApplicationRecord

To ease friction in that case, this also adds a generator for
ApplicationRecord.