Fix pipeline author for Slack and use pipeline id for pipeline link
[Context](https://gitlab.slack.com/archives/runner-builds/p1479238493000018)
Pipeline Slack message:
> gitlab-org/gitlab-ci-multi-runner: Pipeline 8bed2a3b of fix/handle-failed-state-while-patching-trace branch by failed in 1389 seconds
Quotes:
> @godfat Take a look at these notifications, they seems to be missing author and should probably use Pipeline ID instead of SHA
See merge request !7506
Also add Service#event_names so that we don't have to hard code all
event names in ServiceParams. Note that I don't know why we want to
call plural on issue_event and merge_request_event so that we still
need to hard code them for issues_event and merge_requests_event.
See app/helpers/services_helper.rb for those special rules.
Supports four different event types all bundled under the "note" event type:
- comments on a commit
- comments on an issue
- comments on a merge request
- comments on a code snippet