Add hidden config switch to disable code integrity checking

This adds a hidden config flag that allows somebody to disable the code integrity check. If `integrity.check.disabled` is set to `true` in the config file:

1. The integrity check functions will return always an empty result
2. The integrity check is not performed when installing apps
3. The integrity check is not performed when updating apps
4. The integrity check is not performed when updating the core

Furthermore this adds support for a list of channels that the code checker will run on. At the moment this is only stable because I didn't want to break any build scripts that we have. Once we have a proper CA setup and updated the build process to sign the releases we can add the RC, alpha, beta as well as daily releases. So everything except "git" basically.