Task to generate new application secret key base

When a new install is done, generate new, long secret keys for the app, and at any time thereafter.  This adds a further layer of security.

CYB-175 https://jira.ets.berkeley.edu/jira/browse/CYB-175

Running this task will be a required part of a new deploy, or the publicly checked-in secret_key_base will be used.   It won't need to be run after every deploy, only if there's a question of a compromise of the secret.