Replace message status by something meaningful in the code

[antonykamp]

Is your feature request related to a problem? Please describe.
The status of a shopping list (pending, accepted, archived) equals a simple number. Without knowing the meaning behind it, the code is hard to understand.

Describe the solution you'd like
It would be great, if we could handle the status as strings (pending, accepted, archived) in the code and as numbers in the database (1, 2, 3). That sounds like enums to me :)

Describe alternatives you've considered
Mentioning it in the README or CONTRIBUTING is another idea, but it isn't a consistent solution.