Merge branch 'ericy_ts/protected_branches_api' into 'master'

app/models/concerns/protected_branch_access.rb

changelogs/unreleased/ericy_ts-protected_branches_api.yml

doc/api/README.md

doc/api/branches.md

doc/api/protected_branches.md

lib/api/api.rb

lib/api/branches.rb

lib/api/entities.rb

lib/api/protected_branches.rb

spec/factories/protected_branches.rb

spec/requests/api/protected_branches_spec.rb