Backport API to v3

doc/api/v3_to_v4.md

lib/api/api.rb

lib/api/templates.rb

lib/api/v3/boards.rb

lib/api/v3/branches.rb

lib/api/v3/labels.rb

lib/api/v3/repositories.rb

lib/api/v3/system_hooks.rb

lib/api/v3/tags.rb

lib/api/v3/users.rb

spec/requests/api/v3/boards_spec.rb

spec/requests/api/v3/branches_spec.rb

spec/requests/api/v3/labels_spec.rb

spec/requests/api/v3/repositories_spec.rb

spec/requests/api/v3/system_hooks_spec.rb

spec/requests/api/v3/tags_spec.rb

spec/requests/api/v3/users_spec.rb