use property gateway_charge to calculate amount in default currency
fix another currency conversion bug
fix missing currency conversion
use gateway_charge property in smsbillables unit tests
update casexml ref
add regression test for gateway_charge
generate a Currency object with a random exchange rate
Merge pull request #3695 from dimagi/billable-currency-conversion

Billable currency conversion
Update to vellum:f002e91449
Merge remote-tracking branch 'origin/80598-case' into autostaging
Merge branch 'multimedia-case-properties' into autostaging
Merge remote-tracking branch 'origin/cache-locations' into autostaging
Merge remote-tracking branch 'origin/hq-dropdown' into autostaging
Merge remote-tracking branch 'origin/refactor-domain-copy2' into autostaging
Merge remote-tracking branch 'origin/dynamic-links' into autostaging
Merge remote-tracking branch 'origin/offline-install-multimedia' into autostaging
Merge remote-tracking branch 'origin/logos' into autostaging
Merge remote-tracking branch 'origin/109909-case-performance' into autostaging
Merge remote-tracking branch 'origin/switch_to_site_code' into autostaging
Merge branch 'vellum-requirejs' into autostaging