Make all Serializers receive `current_user` instead `user` [EE]

Merged Oswaldo Ferreir requested to merge enforce-current-user-usage-on-entities-ee into master passes current_user to the MergeRequestSerializer, which passes current_user the it's Entity leafs (e.g. PipelineSerializer). The problem is that new leafs were added while wasn't merged, causing conflicts and more overhead about current_user or user usage.

This MR makes all entities receive current_user, avoiding errors and overhead.

CE MR: gitlab-ce!11254

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