Feature: Global Config helper (#844) (#845)

* Added a global config helper to easily access installation/global configs
  * this will fetch the keys from cache with fallback to DB on cache miss
  * ability to query multiple keys simultaneously
  * interface to delete the existing global config cache
* Added tests for this new helper module

.rubocop.yml

@@ -30,6 +30,7 @@ Style/GlobalVars:
   Exclude:
     - 'config/initializers/redis.rb'
     - 'lib/redis/alfred.rb'
+    - 'lib/global_config.rb'
 Metrics/BlockLength:
   Exclude:
     - spec/**/*