Add optional hCaptcha support

Fixes #1649

This requires setting `HCAPTCHA_SECRET_KEY` and `HCAPTCHA_SITE_KEY`, then
enabling the admin setting at
`/admin/settings/edit#form_admin_settings_captcha_enabled`

Subsequently, a hCaptcha widget will be displayed on `/about` and
`/auth/sign_up` unless:
- the user is already signed-up already
- the user has used an invite link
- the user has already solved the captcha (and registration failed for another
  reason)

The Content-Security-Policy headers are altered automatically to allow the
third-party hCaptcha scripts on `/about` and `/auth/sign_up` following the same
rules as above.

app/views/about/_registration.html.haml

@@ -21,6 +21,9 @@
     .fields-group
       = f.input :agreement, as: :boolean, wrapper: :with_label, label: t('auth.checkbox_agreement_html', rules_path: about_more_path, terms_path: terms_path), required: true, disabled: closed_registrations?
 
+    .fields-group
+      = render_captcha_if_needed
+
     .actions
       = f.button :button, sign_up_message, type: :submit, class: 'button button-primary', disabled: closed_registrations?
 

app/views/admin/settings/edit.html.haml

@@ -42,7 +42,10 @@
 
   .fields-group
     = f.input :require_invite_text, as: :boolean, wrapper: :with_label, label: t('admin.settings.registrations.require_invite_text.title'), hint: t('admin.settings.registrations.require_invite_text.desc_html'), disabled: !approved_registrations?
-  .fields-group
+
+  - if captcha_available?
+    .fields-group
+      = f.input :captcha_enabled, as: :boolean, wrapper: :with_label, label: t('admin.settings.captcha_enabled.title'), hint: t('admin.settings.captcha_enabled.desc_html')
 
   %hr.spacer/
 

app/views/auth/registrations/new.html.haml

@@ -38,6 +38,9 @@
   .fields-group
     = f.input :agreement, as: :boolean, wrapper: :with_label, label: whitelist_mode? ? t('auth.checkbox_agreement_without_rules_html', terms_path: terms_path) : t('auth.checkbox_agreement_html', rules_path: about_more_path, terms_path: terms_path), required: true
 
+  .field-group
+    = render_captcha_if_needed
+
   .actions
     = f.button :button, @invite.present? ? t('auth.register') : sign_up_message, type: :submit