Integration support should be globally configurable

[DivineOmega]

Currently Slack integration can only be defined per URL in the urls.json file. In situations where the same Slack integration details are used for all sites, this results in lots of duplication of Slack configuration parameters.

A new configuration file should be created (integrations.json) to provides global configuration for integrations. These settings could then be overridden on a per URL basis if required.