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/helpers/admin/settings_helper.rb

@@ -8,4 +8,8 @@ module Admin::SettingsHelper
     link = link_to t('admin.site_uploads.delete'), admin_site_upload_path(upload), data: { method: :delete }
     safe_join([hint, link], '<br/>'.html_safe)
   end
+
+  def captcha_available?
+    ENV['HCAPTCHA_SECRET_KEY'].present? && ENV['HCAPTCHA_SITE_KEY'].present?
+  end
 end