add checkbox to ffxiv price alert generator for "dc_only" option

[Alex Cohen]

need an option on this page to add "dc_only": true to the json like the following if checked, this should be unchecked by default

{
  "home_server": "Famfrit",
  "user_auctions": [
    { "itemID": 21794, "price": 100000, "desired_state": "below", "hq": false }
  ],
  "dc_only": true
}