Avoid having an image proof with multiple price dates

[raphodn]

Story

Similar to #244, for image proofs (price tags, receipt), the date should be consistent on each related price.

Applicable only for PRICE_TAG & RECEIPT proof types.

How

A check before price creation, or a background task to add flags to proofs with multiple price dates ?

• we could store a proof date when the first price is created ? Proof: store an optional date #322
• what if the user then wants to edit the price date ?
• or extract the date from the proof image ? Extract data from proof images #107