feat: Add the ability for a super admin to define account limits (#3946)

Fixes: #3773

app/fields/enterprise/account_limits_field.rb

@@ -0,0 +1,7 @@
+require 'administrate/field/base'
+
+class Enterprise::AccountLimitsField < Administrate::Field::Base
+  def to_s
+    data.present? ? data.to_json : { agents: nil, inboxes: nil }.to_json
+  end
+end