Generate secret for webhook from personal token

[jgable]

We should hash the token using SHA256 and pass it as a secret to be posted with GitHub Events to protect against random people POSTing data to the server.

If necessary, we should also verify the secret in the handleEventPost method.

[parkr]

:+1:

[jgable]

I'll try to take a look at this this week if I can find time. Let me know if you'd like to take a shot at it yourself so we don't overlap.

[parkr]

I can probably look at this tonight!