Improve code related to the API sudo feature

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• The API allows admin users to impersonate any other user by using the sudo feature
• The code for this feature is scattered across a few methods in lib/api/helpers.rb
• Background for the creation of this issue: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10842#note_28348788

Potential improvements

• Rename the initial_current_user method. Currently, during a sudo call, initial_current_user points to the admin user, and current_user points to the user they are impersonating.

• The sudo-related methods and constants in API::Helpers could be moved to a separate class/module/concern.

/cc @rymai @jamedjo