Add notes notifications to services

This merge request builds on top of a few other merge requests:

• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/237
• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/350

The purpose of this merge request is to add service notification support when a user comments on one of the following:

• commits
• issues
• merge requests
• code snippets

Currently HipChat and Slack are only supported. The admin can select which events to toggle on or off.

Questions

All note events are bundled under the note event type. Is this okay, or does it make more sense to have individual events and/or change the object_type attribute in the hook data?

Also, does it look better to link the entire merge request #X as opposed to just #X? I like the former.

Before

After

Slack

HipChat