handle attribute external_id (fms_id) as a special case in Open311 send-reports

[davewhiteland]

Proposed change: when sending a problem report over Open311 that has an attribute['fms_id'] , automatically populate that attribute with the report's id.

FMS-endpoint expects the FMS report ID to be provided in an attribute called external_id (although there's a strong case for this being fms_id -- see mysociety/fms-endpoint#19 so there's no risk of a nameclash with some other use of external_id).

Currently we're doing this by switching on the cobrand (fixmybarangay) which means FMS-endpoint doesn't quite work as expected with default FMS installs.

[davewhiteland]

If this change is implemented, then FMS-endpoint can provide the attribute in its service discovery responses (see mysociety/fms-endpoint#20 ) -- then FMS + FMS-endpoint would exchange FMS ids nicely, automatically.

[dracos]

Dealt with, as fixmystreet_id (or overrideable id field name in PopulateServiceList), by #1499.