Display gray checkbox against shopping list items whose quantities have been updated and invalidated

[jayaddison]

Is your feature request related to a problem? Please describe.
As described in #148, a user may schedule a recipe (meal) multiple times on their meal planner calendar.

It's possible that when they are adding a new scheduled meal that some of the ingredients/products from that meal already exist in their shopping list - and may have been previously checked to indicate that they've been purchased.

We can't know for sure what quantity of the ingredient the user may have purchased, so we can't determine whether to untick the checkbox or leave it checked.

Describe the solution you'd like
Would should leave the shopping list ingredient checkbox ticked, but also gray it out (or somehow otherwise highlight it) to indicate that the ingredient purchase state has been invalidated by meal plan changes.

Additional context
This issue has branched off from #2.

[jayaddison]

NB: Invalidation only needs to occur when the scheduled requirements for that ingredient have increased -- if they have decreased then it should be safe to assume that the product has still been purchased in sufficient quantity.