Add link to GNUPG Key
Correctly render non_field_errors on forms
Add a sanity check on the nutritional values of an ingredient according
to the following rules

- 1g of protein: 4kcal
- 1g of carbohydrates: 4kcal
- 1g of fat: 9kcal
Let users submit ingredients to the database

The workflow for the process is identical to the one for submitted exercises