Issue #8: item checkbox

Description

Added checkbox to every item in list. User able to check item as purchased and date of purchase is saved to db as a purchacedDate. The date is saved as a number in milliseconds.
When list is loaded the item stays checked if less then 24 hours since purchased date and then it is unchecked.
Added functionality to allow user to uncheck box, in this case the purchasedDate reset to null.

Extras:

Refactored AddItem.js: created separate function to check for duplicate (duplicateCheck), to remove punctuation (removePunctuation)
Changed the way how we save items in db: items saved under normalized names as a key and now we able to check the db for existing item using this key. We don't need to fetch the whole list of items.

Closes #8

User is able to tap a checkbox or similar UI element to mark an item in the list as purchased
Item should be shown as checked for 24 hours after the purchase is made (i.e. we assume the user does not need to buy the item again for at least 1 day)

Run the application, navigate to listView and confirm checkboxes are next to items.
Click one of the checkboxes, then refresh the app and confirm that checkbox remains checked.
Navigate to firebase, find your shopping list and confirm that purchasedDate for this item was saved as a number.
To test the 24 hour function: change the purchasedDate for checked item in firestore to 1643653903254 (Jan 31 in js in milliseconds) and return to your shopping list and confirm that checkmarked has been removed.