Revert "Remove HipChat integration from GitLab"

This was picked into the 11.9.8 patch release: https://about.gitlab.com/2019/04/11/gitlab-11-9-8-released/

We intend to remove this again in future: https://gitlab.com/gitlab-org/gitlab-ce/issues/60279

For customers upgrading and wanting to continue to use self-hosted HipChat, these are the scenarios:

Already upgraded to 11.9.X (where X < 8)

HipChat configurations were deleted from the database in 11.9.0. The only way to recover them is from backup, but they can be re-added.

Not yet upgraded to GitLab 11.9

No change for now, HipChat will continue to work.

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This reverts https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22223. See https://gitlab.com/gitlab-org/gitlab-ce/issues/60042 for why.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/60042.