Webhook: refactor and solve alerts ending up requiring additional requests

[davidor]

Issue reported by @unleashed:

backend requires further activity to "flush" alerts to system.

The whole alerts code (event_storage.rb particularly) is rather low quality. We should improve the design (avoiding the problem above) and fix the awful code that handles the webhook. A Webhook class should be created (nice if it also can handle custom Host headers), and used, hopefully without the need to re-create the client and being configurable for timeouts with proper error handling.

The "proper error handling" part is avoiding horrible stuff like rescue => e; notify(); end which caused NoMethodError, NameError and LoadError among others to inflict infinite pain to me for hours.