Added X-GitLab-Event header for web hooks

CHANGELOG

app/controllers/admin/hooks_controller.rb

app/models/hooks/service_hook.rb

app/models/hooks/web_hook.rb

app/models/project.rb

app/services/system_hooks_service.rb

app/services/test_hook_service.rb

app/workers/project_web_hook_worker.rb

app/workers/system_hook_worker.rb

doc/system_hooks/system_hooks.md

doc/web_hooks/web_hooks.md

lib/api/system_hooks.rb

spec/models/hooks/service_hook_spec.rb

spec/models/hooks/system_hook_spec.rb

spec/models/hooks/web_hook_spec.rb