Merge branch 'captcha' into 'develop'

config/config.exs

config/config.md

config/test.exs

lib/pleroma/application.ex

lib/pleroma/captcha/captcha.ex

lib/pleroma/captcha/captcha_service.ex

lib/pleroma/captcha/kocaptcha.ex

lib/pleroma/web/router.ex

lib/pleroma/web/twitter_api/controllers/util_controller.ex

lib/pleroma/web/twitter_api/twitter_api.ex

test/captcha_test.exs

test/support/captcha_mock.ex