As a user I want to add the dishes in specific categories.

## Dish categories

Currently, dishes are not categorized. So, searching for and sorting of dishes is only possible by dish name. In the future both should be enabled for categories as well. In preparation to that, restaurant owners have to be provided with the functionality to classify their dishes into predefined categories. The category a dish belongs to has to be visualized in the dish overview for both the restaurant owner and hungry person, then.

# Checklist
- [x] Create CATEGORIES table in database
- [x] Create foreign key in table DISHES to table CATEGORIES
- [ ] Provide SQL script containg database changes
- [ ] Update test dump to reflect changes on database
- [x] Create categories model file
- [x] Adapt dishes model file
- [ ] Modify menu view, so that category of a dish is displayed both for the restaurant owners and hungry people
- [ ] Modify edit view, so that when a dish is created a category can be selected and when a dish is modified that the category can be changed

# Questions
- Who is responsible for defining categories? Is it the administrator? Should restaurant owners be able to define categories themselves?
- Where shall we store SQL scripts? Shall we attach them to pull requests?