#1158 Allow multiple prices per publication

[Ahasuerus]

As per http://www.isfdb.org/wiki/index.php/ISFDB:Community_Portal#Redesigning_prices (June 2018):

Each publication record should allow an unlimited number of prices. This will be accomplished by converting the "Price" field of publication records to a "multi-field", which will use the standard "+" mechanism for adding additional prices.

Within editing forms, each price row will contain three sub-fields per price:
"Currency": a drop-down list of all currencies supported by ISFDB, e.g. "£", "€", "$", "¥", "Kč" or "FF". This drop-down list will include additional information to facilitate the data entry process, e.g. "C$ - Canadian dollars" or "A$ - Australian dollars".
"Value": a free text field containing values like "24.00", "60,000", "2/6" etc. Blank values are allowed if we know the currency, but not the price.
"Comment": a free text field for optional explanatory information, e.g. "(Germany)"
The look-and-feel will be similar to that of the "External ID" multi-field, which contains two sub-fields (External ID type and External ID value) per row.

Within each publication, prices should be displayed in "priority order". For example, a UK book which has one price for the UK, another price for Australia and a third price for New Zealand should display the UK price first. Note that this is different from the behavior of other "multi-fields", e.g. the order of authors within publications/titles is not enforced while external IDs are displayed alphabetically.

Each displayed price should contain four components:

    1. The price value, e.g. "24.00", "60,000" or "2/6"
    2. The currency symbol, e.g. "£", "€", "$", "¥", "Kč" or "FF". The currency symbol should be displayed before or after the price value depending on the currency. Some symbols should be adjacent to the price value while other symbols should be separated with a space. Some currencies, like the old UK currency and "other", will have an empty displayed currency symbol.
    3. The optional "price comment" value described above.
    4. Currency-specific text displayed as a mouse-over bubble, similar to the way we display mouse-over information for publication formats. It will contain information about the currency, including the countries where it is/was used and any applicable ISO-4217 codes (some currencies have had multiple ISO-4217 codes and some older ones have none.) It will also be used to explain the formatting of obscure and unusual currencies like the old UK system.

Prices whose currency is not in the drop-down list of supported currency symbols will be entered as follows:
The editor will select "other" in the list of currency symbols
The editor will enter the price in the "value" field
The editor will enter the currency symbol and any additional explanatory information in the "comment" field
Once a critical mass of publications with the same unsupported currency symbol has been reached, the currency will be added to the list of supported currencies. At that point all affected publications will be manually updated: the currency symbol will be moved from the "comment" field to the "symbol" field and the remaining "comment" information will be reviewed to see if it's still needed.

At implementation time, common currencies will be converted to the new design automatically. There will be one or more cleanup reports to handle uncommon currencies, typos, etc. There will be additional cleanup reports to find publications with price information in Notes.