Protected Tags backend review changes

app/controllers/projects/protected_branches_controller.rb

app/controllers/projects/protected_refs_controller.rb

app/controllers/projects/protected_tags_controller.rb

app/helpers/branches_helper.rb

app/models/concerns/protected_branch_access.rb

app/models/concerns/protected_ref.rb

app/models/concerns/protected_tag_access.rb

app/models/project.rb

app/models/protectable_dropdown.rb

app/services/protected_branches/update_service.rb

app/services/protected_tags/update_service.rb

app/views/projects/branches/_branch.html.haml

app/views/projects/protected_branches/show.html.haml

app/views/projects/protected_tags/show.html.haml

...s/unreleased/18471-restrict-tag-pushes-protected-tags.yml

db/migrate/20170309173138_create_protected_tags.rb

lib/gitlab/checks/change_access.rb

...ontrollers/projects/protected_branches_controller_spec.rb

spec/controllers/projects/protected_tags_controller_spec.rb

spec/features/protected_branches/access_control_ce_spec.rb