Enforce Y-m-d as date format for date fields

[snipe]

These changes are mostly for the API, since the GUI handles dates via date picker and mostly won't let you make mistakes there. This came up because a discord user was using the API and tried to send 6/2/2022 as a purchase date, which was passing the looser date validation (since it is a valid date) but was being inserted into mysql very weirdly, since MySQL doesn't know what to do with that date, as it's expecting YYYY-MM-DD. This at least kicks back an error to prevent badly formatted data from creating inaccurate results.

[what-the-diff]

• Change the date format in Accessory, Asset, Component and Consumable models
• Add purchase_date field to License model
• Add expiration_date and termination_date fields to License model
• Fix start/end dates for User Model (change from nullable|after:start_data)