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/locales-glitch/en.yml

@@ -2,6 +2,9 @@
 en:
   admin:
     settings:
+      captcha_enabled:
+        desc_html: Enable hCaptcha integration, requiring new users to solve a challenge when signing up. Note that this disables app-based registration, and requires third-party scripts from hCaptcha to be embedded in the registration pages. This may have security and privacy concerns.
+        title: Require new users to go through a CAPTCHA to sign up
       enable_keybase:
         desc_html: Allow your users to prove their identity via keybase
         title: Enable keybase integration