Expand "Secrets" concept

Secrets are standins for user-supplied values. The idea is that a person could reference a secret that has a known name, and trust that the appropriate value will be available at runtime. For example (pseudocode):

# in task
def run(self):
    log_in_to_some_service(Secret('email'), Secret('password'))

The idea is that when the flow is running, calling Secret(x) returns the value of the secret called “x”, which must be provided or available somehow (so perhaps tasks must declare what secrets they depend on?).

Will require server coordination cc @cicdw @joshmeek

[jlowin]

My gut is that a secret is an account-level (or possibly project-level, but that introduces complications) variable with a string key unique to that account and a json-able value.

[cicdw]

Closed via #323