Input filters

The attached patch adds the input filter from 7.x-1.x in a slightly improved form: better UI texts, filter tips, and cleaner and more flexible code, altered to use the new conversion API from #1848966: Pluggable currency conversion, on which this patch of course depends.

Patch depends on #1848966: Pluggable currency conversion.

There are now two input filters: one to insert exchange rates, and one to convert amounts from one currency to another. It has tests too.

There is just one input filter, which converts amounts. If the amount is not given, or if it is 1, then it functions as an exchange rate display.

Amounts need to be parsed using bartfeenstra/currency, and I'd like to add a filter to format/localize currency amounts.

The patch adds two input filters:

• [currency:currency_code_from:currency_code_to], [currency:currency_code_from:currency_code_to:amount] to convert amounts from one currency to another. This filter is identical to the one in 7.x-1.x, with the exception that the amount may be in a localized format.
• [currency-localize:currency_code:amount] to display an amount in the current locale. This input filter is not cacheable.