Slash Commands for HipChat

Description

Similar to the support for Chat Ops in Slack (gitlab-ce#22182) and Mattermost (gitlab-ce#22540), it would be nice to have such support for HipChat.

Proposal

Build a RESTful endpoint for HipChat to post to, to allow slash commands to function.

https://blog.hipchat.com/2015/02/11/build-your-own-integration-with-hipchat/

Links / references

• https://www.hipchat.com/docs/apiv2
• https://developer.atlassian.com/hipchat/guide

Documentation blurb

Some organizations use HipChat to collaborate among their teams, and they would also benefit from Chat Ops support form GitLab.

To support that, a special RESTful endpoint can be configured and exposed from GitLab to allow HipChat to POST to it to notify GitLab of new slash commands.

This is configured in the "Integrations" section of the project.