only retrain when enough new data is available

[camallen]

if there is no or not much training data then avoid kicking off a retraining job doesn't add much value. We should have a min threshold for starting the training batch jobs.

[camallen]

Added notes on how and where in the data cycle this could be implemented

kade/app/sidekiq/retrain_zoobot_job.rb

Lines 29 to 33 in a3271fa

	# this is where we could intercept the training job submission
	# to avoid a training run if there isn't enough data for a viable model
	# one idea would be to check the number of rows in the training data export attached file
	# or even better we store the number of exported rows in the training data export model
	# https://github.com/zooniverse/kade/issues/62