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.

config/settings.yml

@@ -77,6 +77,7 @@ defaults: &defaults
   show_domain_blocks_rationale: 'disabled'
   outgoing_spoilers: ''
   require_invite_text: false
+  captcha_enabled: false
 
 development:
   <<: *defaults