More consistent event names
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We're storing as `services.issues_events` and `services.merge_requests_events`, that is issues and merge_requests are plural. However, the event name is singular like `issue` and `merge_request`. On the other hand, all other event names are all singular.
This caused that we need some special rules in a number of places, namely the view form.
gitlab-ce!5861 is an attempt to make this consistent without changing the data. However, it seems it's causing even more special rules. Please see the comment at: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5861#note_14057068
So to solve this once and for all, we should probably rename the columns, namely use `services.issue_events` and `service.merge_request_events` then we could remove all the special rules.
This issue is created in request in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5620#note_14081185
/cc @felipe_artur @grzesiek
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