distinguish SIGINT from errors that need to be reported
execute cells at a particular version

if you queue a cell for execution and then delete or change it,
the version that was queued is still run

eliminate the horrrible cell_eval that took R code
now the only difference for unauthenticated it checking if published

get rid of cell_controller.execute
instead cell_model.get_execution_info captures the partname and version

record_cell_execution now takes the json rep
(because the cell_model may not exist anymore)