Add ability to provide custom comment when Ack-ing a service alert. Provide a fix for issue #50.

[brennentsmith]

This PR provides the ability for users to submit a custom comment when Ack-ing a service alert - we use this heavily at Ookla so the team knows why and who is handling an alert. If the field is left blank, the comment will fall back onto the default nagdash comment. Screenshot of UI change is here: http://ookla.d.pr/17KI1

Also provide a fix for: https://github.com/lozzd/Nagdash/issues/50