Support custom action for automation rules with webhook + api

[stsewd]

Currently, the actions to be run are only defined in code

https://github.com/readthedocs/readthedocs.org/blob/master/readthedocs/builds/automation_actions.py

Users can't set a custom action. We don't want to execute arbitrary code neither. But we can offer users to set a webhook that will be called on a match. Users can use apiv3 to do whatever they want to the version or project.

Why don't just use api?

Because users don't have a way to know when the version is created on rtd (nor the final slug). They can only know when the version is created on github.

Examples

• Match: semver
• action: webhook
• value: https://example.com/webhook

The webhook receives a payload with the project and the matched version. Then the user can trigger a build or deactivate the version or modify other versions, anything.