Decouple ItemManager methods from populateItems method

[ash-bergs]

          This work put me over 10 touched files (13) - but I aimed to do a few things:

• Abstract areas where we're directly hitting the database and manipulating items into a single place, the DatabaseService
• Remove methods that were really just utility functions that didn't need to be on ItemManager at all
• Make the ItemManager class easier to read and breakdown

There are still areas for improvement, like the coupling of populateItems on all the methods. This can be fixed, but will increase changed files even more so I'll quickly do it following this.

_Originally posted by @ash-bergs in https://github.com/ash-bergs/pantry-pal/pull/43#discussion_r1753093059_

[ash-bergs]

We should just call itemManager.populateItems in the places we call the other methods, instead of inside the methods themselves.

[ash-bergs]

As a part of this we can cleanup some methods as well, like add/remove singular items will basically just call the database service. I'm gonna noodle on the pattern for that for a minute, I wonder if I've flown too close to the sun creating the database service lol - but it was a decision made to give us clear division between database/data logic and ui logic.