Update Asana integration to use new gid field.
Review Request #10651 — Created July 29, 2019 and submitted
Because they're approaching the limits of integers, Asana has been
migrating their API to use a string-based ID field for all tasks. This
change updates our integration to use that new ID.
Used the Asana field to link to several tasks. Saw that the new ones
used the new gid field, and that old items still linked correctly and
updated the stored data when saved.