Change CAPTCHA handling to be only on email verification

This simplifies the implementation considerably, and while not providing
ideal UX, it's the most flexible approach.

config/settings.yml

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