Fix for JENKINS-17503 by adding an excluded committer list to email-ext global config

This pull request adds an excluded committer list to the email-ext plugin's global configuration user interface so the account names can be added that will be prevented from receiving emails by this plugin. I see a preference for a more global solution - but since this code has been running in our group for almost a year and it's already written - even if it's not used it could help the conversation.