Add button in Inventory should allow you to add any count of item to existing items

[ethanlee16]

Is your feature request related to a problem? Please describe.
The views for adding inventory to an item that already has some inventory and adding inventory to an item that does not have inventory are currently separate, but they should be merged to be a part of the same view.

It would be nice to have the Plus floating action button in the Inventory tab be a quick way to add inventory counts to any item!

Describe the solution you would like to see

Within this screen, all items (not just those of item count = 0) should be visible for selection. It might also be nice for the input to be for "an additional amount to add" as opposed to setting the count directly.

cc: @julianrkung