Fix #172 -- Add dietary preferences to ticket model

[zerok]

@Markush2010: how should the dietary preferences be exported?

[zerok]

@Markush2010 Something like that?

[
 {
  "profile": null, 
  "status": null, 
  "trainings": null, 
  "uid": null, 
  "tags": null, 
  "tshirt": null, 
  "sponsor": null, 
  "id": 71, 
  "name": "Horst Gutmann", 
  "days": null, 
  "dietary_preferences": [
   "vegan", 
   "gluten-free"
  ], 
  "organization": null
 }, 
 {
  "profile": null, 
  "status": null, 
  "trainings": null, 
  "uid": null, 
  "tags": null, 
  "tshirt": null, 
  "sponsor": null, 
  "id": 74, 
  "name": "Horst Gutmann", 
  "days": null, 
  "dietary_preferences": [
   "vegan", 
   "halal"
  ], 
  "organization": null
 }
]

[MarkusH]

I don't think it needs to be exported at all. If it's needed, we can add it later.