Add API call to flush caches of a namespace or project
We are doing this with a rails console for when the cache gets poisoned.
We should just formalize it with an API call so we can do it through chatops.
The calls we are using are like this:
projects = Namespace.find_by(name: 'namespace')
projects.map(&:repository).map(&:expire_all_method_caches)
cc/ @DouweM