Proof: link a date, location & currency

[raphodn]

Story

In the backend we're adding new fields to the Proof model

• date : Proof: store an optional date open-prices#322
• location : Proof: store an optional location open-prices#323
• currency : Proof: store an optional currency open-prices#336

We can now improve the price add form to ask first for the proof + date + location + currency.
This will simplify price addition afterwards (pre-fill form, quality check on price data, ...)

This is valid for RECEIPT & PRICE_TAG proofs

[teolemon]

Next, we need Open Prices to aggressively nudge you if you haven't inputed prices on all of your receipts :-)

[raphodn]

@monsieurtanuki in the next few days i will focus on integrating these new Proof fields.

• the UI will change a bit, we'll be asking first the proof data, and then allow adding prices (slightly more proof-oriented)
• if a user selects an existing proof with these info, the form will be pre-filled (and the fields set as readonly)
• new info will be displayed under each proof, and possibility to order & filter as well
• futur step will be to send these proofs to Proof: send to Google Cloud Vision, and store the matching JSON output open-prices#320 and get extra data (and one day pre-fill these info automatically)