Replace dayOffset with WeekDay Enum

[jmesserli]

Currently, the API has to be accessed using annoying dayOffset values. A value of 1 means, that you want to see the menu of the day after today.
This is no longer the way the website is queried and can be confusing because the meal plans can be available before the next week has begun.

The new way would be /api/restaurant/:id/meal/[monday-friday]