Merge branch '26325-system-hooks' into 'master'

app/controllers/admin/hooks_controller.rb

app/models/hooks/system_hook.rb

app/models/hooks/web_hook.rb

app/views/admin/hooks/_form.html.haml

app/views/admin/hooks/index.html.haml

app/workers/post_receive.rb

changelogs/unreleased/26325-system-hooks.yml

...170503023315_add_repository_update_events_to_web_hooks.rb

db/schema.rb

doc/system_hooks/system_hooks.md

lib/api/entities.rb

lib/gitlab/data_builder/repository.rb

lib/gitlab/git_post_receive.rb

spec/controllers/admin/hooks_controller_spec.rb

spec/lib/gitlab/import_export/safe_model_attributes.yml

spec/models/hooks/system_hook_spec.rb

spec/requests/api/system_hooks_spec.rb

spec/requests/api/v3/system_hooks_spec.rb

spec/workers/post_receive_spec.rb