Change MainItem to Category and Create AddCategory

Story Points: 2

# How?

Right now, the current inventory system isn't very user-friendly. After talking to Jeremy, it was decided that the MainItems seen on the Inventory page would be called Categories, housing the previously called "SubItems" as the actual items, each having a quantity associated.

This issue is to rename the MainItem to Category in all the appropriate places, and to add an AddCategory page that is separate from the AddItem page so that they are separate functions that are semantically different in the eyes of the user.