Consolidate duplicate user data attribute presentation code

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Context

The following discussion from !17306 (merged) should be addressed:

• @jejacks0n started a discussion: (+1 comment)

  I see a similar use over in https://gitlab.com/gitlab-org/gitlab/blob/master/app%2Fviews%2Fshared%2Fissuable%2Fform%2F_metadata_issuable_assignee.html.haml#L5

  Can/should we try to consolidate using this helper?

  Logic being, assignee_sidebar_data makes merge_request optional by defaulting it to nil, but there are no usages currently where merge_request isn't provided.