Propose get days from date functions

[kweeuhree]

Description

This code adds a function getDateLastPurchasedOrDateCreated that takes in two Timestamps: dateLastPurchased and dateCreated. The function will check whether dateLastPurchased can be converted to a date, and if not it will return a valid date that the item was created.
Why adding:

• I think that this function can help to abstract some OR logical checks.
  The name is hefty, so I appreciate any input here!

This code also proposes a getDaysFromDate function, that takes in a valid JS date and returns the amount of days that passed from that date until today.
Why adding:

• Maybe can help simplify the calculatePurchaseUrgency function.

Related Issue

Related to issue 13

Type of Changes

enhancement

Updates

Testing Steps / QA Criteria

Ensured that calculateDateNextPurchased still works with the new abstraction

[github-actions]

Visit the preview URL for this PR (updated for commit 57119a5):

https://tcl-75-smart-shopping-list--pr51-propose-get-days-fro-1f99qlcy.web.app

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