Add update action to /github-events endpoint

[joshsmith]

What's in this PR?

• adds validation of retry param when updating
• adds update_changeset to the event
• adds inclusion validation of the various statuses
• adds update policy
• adds update action to the controller
• adds tests for all of the above

References

Fixes #1168

[joshsmith]

This needs some tests for the webhook handler for the cases where the event already exists and needs to be reprocessed.

[begedin]

Looks fine, but my comment from the ember PR stands. This is an update action, but the user is not really patching a record. They're requesting performance of a non-RESTful action from the system.