Job pay graph should show when "any" currency is selected

[jace]

The job pay graph only shows when a currency is selected. It should show regardless of whether a currency is selected, showing payscales in all currencies.

1. Show a separate line per currency.
2. Allow a base currency to be specified in settings (default INR) and recast all currencies into this, so that the lines show correctly relative to each other.
3. Specify an exchange rate:
   1. Either hardcoded into settings and periodically revised, or
   2. Fetched from a currency exchange service and stored in cache, or
   3. Perhaps somewhat controversially, using purchasing power parity instead of real exchange rate (hardcoded since it won't fluctuate as much).

The third is controversial because it assumes Hasjob is used by Indians seeking to move abroad, instead of being used by Indians to find remote work abroad, since real exchange rates matter more than PPP in the latter context.