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.

.env.production.sample

@@ -285,3 +285,7 @@ MAX_POLL_OPTION_CHARS=100
 # Units are in bytes
 MAX_EMOJI_SIZE=51200
 MAX_REMOTE_EMOJI_SIZE=204800
+
+# Optional hCaptcha support
+# HCAPTCHA_SECRET_KEY=
+# HCAPTCHA_SITE_KEY=