Add ability to set hCaptcha either on registration form or on e-mail validation

Upshot of CAPTCHA on e-mail validation is it does not need to break the in-band
registration API.

app/models/form/admin_settings.rb

@@ -40,7 +40,7 @@ class Form::AdminSettings
     noindex
     outgoing_spoilers
     require_invite_text
-    captcha_enabled
+    captcha_mode
   ).freeze
 
   BOOLEAN_KEYS = %i(
@@ -59,7 +59,6 @@ class Form::AdminSettings
     trendable_by_default
     noindex
     require_invite_text
-    captcha_enabled
   ).freeze
 
   UPLOAD_KEYS = %i(
@@ -83,6 +82,7 @@ class Form::AdminSettings
   validates :bootstrap_timeline_accounts, existing_username: { multiple: true }
   validates :show_domain_blocks, inclusion: { in: %w(disabled users all) }
   validates :show_domain_blocks_rationale, inclusion: { in: %w(disabled users all) }
+  validates :captcha_mode, inclusion: { in: %w(disabled registration-form email-confirmation) }
 
   def initialize(_attributes = {})
     super