Change project cache key to depend on ID instead of full path

Previously, project cache used as part of the namespace the `full_path`,
which included namespace and project slug.

That meant that anytime a project was renamed or transfered to a
different namespace, we would lose the existing cache. This is not
necessary, nor desired.

I've prefixed cache key with `project:` to make it easy to find in redis
if necessary as well as make it possible to purge all project related
cache.

I've also switched the cache key type to go after the initial namespace
and not before.

lib/gitlab/repository_cache.rb

@@ -7,13 +7,13 @@ module Gitlab
 
     def initialize(repository, extra_namespace: nil, backend: Rails.cache)
       @repository = repository
-      @namespace = "#{repository.full_path}:#{repository.project.id}"
+      @namespace = "project:#{repository.project.id}"
       @namespace = "#{@namespace}:#{extra_namespace}" if extra_namespace
       @backend = backend
     end
 
     def cache_key(type)
-      "#{type}:#{namespace}"
+      "#{namespace}:#{type}"
     end
 
     def expire(key)