Add webhook controller actions

I had considered making a WebhooksController that inherited from `ActionController::Base`, but then we lose out on any security measures the user has put in `ApplicationController`. But inheriting from ApplicationController couples us to the user's naming: what if they wanted webhooks for an `Admin::ApplicationController`?

Since we'll need create a concrete `*WebhooksController` in each app anyway to disable various security measures (like auth tokens), it seems better to just make a module of methods we want to mixin to that controller for convenience.

On the plus, this also means that we don't need to be tied to Rails' ActionPack at all, so I nuked it from the gemspec.