[add] added maximum client and demo client protection

- default demo clients can no longer be modified/deleted when in demo mode
 - increased default demo reset to 60 minutes

app/models/client.rb

@@ -3,6 +3,8 @@
 class Client < ApplicationRecord
 
   def self.new_client(params)
+    return nil if count >= RynDNS::Application::MAX_CLIENTS
+
     perm_string = params[:permission] || 'w' # create write client by default
     perm_rwx = Client.parse_perm_string(perm_string)
 
@@ -28,7 +30,7 @@ class Client < ApplicationRecord
   def gen_api_key
     api_key = SecureRandom.hex(32)
     self.api_key = api_key
-    "#{RynDNS::Application::API_KEY_PREFIX}.#{self.name_id}.#{api_key}"
+    "#{RynDNS::Application::API_KEY_PREFIX}.#{name_id}.#{api_key}"
   end
 
   def api_key