Set up GitLab Slack App on GitLab.com

I need to set Client ID, Client Secret and, Verification token for GitLab Slack App in admin area but not to enable the service itself. We will need to enable it later, after successful testing.

The work is needed in a scope of https://gitlab.com/gitlab-org/gitlab-ee/issues/2629

/cc @omame @pcarranza

changed the description

assigned to @victorcete

changed milestone to %WoW ending 2017-07-18

added unblocks others unscheduled labels

Done! (and kept the feature disabled as requested)

closed

@victorcete Thanks. I will let you know when we can enable it