Use reCaptcha when an issue identified as spam

app/assets/stylesheets/pages/issues.scss

app/controllers/concerns/spammable_actions.rb

app/controllers/projects/issues_controller.rb

app/controllers/registrations_controller.rb

app/models/concerns/spammable.rb

app/services/issues/create_service.rb

app/services/spam_service.rb

app/views/admin/spam_logs/_spam_log.html.haml

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

app/views/devise/shared/_signup_box.html.haml

app/views/projects/issues/verify.html.haml

changelogs/unreleased/21518_recaptcha_spam_issues.yml

...ate/20170206071414_add_recaptcha_verified_to_spam_logs.rb

db/schema.rb

lib/gitlab/recaptcha.rb

spec/controllers/projects/issues_controller_spec.rb

spec/controllers/registrations_controller_spec.rb

spec/features/issues/spam_issues_spec.rb

spec/services/issues/create_service_spec.rb

spec/services/spam_service_spec.rb