Allow incrementing/decrementing servings for each meal on the meal planner individually

[jayaddison]

Is your feature request related to a problem? Please describe.
It's valid and possible to schedule the same recipe (meal) for various days on the meal calendar.

For various use cases -- for example, if a user has invited friends over for dinner on a certain day -- it makes sense to allow the number of servings for a meal to be modified individually.

Describe the solution you'd like
Add small increment/decrement buttons on the meal planner page next to each meal that allow the user to increase or decrease the number of servings for that meal.

The updated servings values should be reflected in the quantities listed on the shopping list.

Some application data model refactoring may be required to achieve this cleanly.

Additional context
This issue has branched off from #2.