Add custom extractor for sorting for objects.

[bharathcs]

Fixes #353

[ganhongyao]

Some design-related comments.

Again, I feel that this is a temporary solution and we should probably move to sorting in the backend if we continue to have to make such code changes in future. Hmm

[bharathcs]

Thanks for the review Hong Yao, I've cleared up the comments you've mentioned. As for the design comment, we resolved this offline in tg, but for leaving a record of our discussion here:

Some design-related comments.

Again, I feel that this is a temporary solution and we should probably move to sorting in the backend if we continue to have to make such code changes in future. Hmm

No point offloading the sorting to the backend if the frontend needs to refetch all of the data over and over again with every change in sort order (by clicking a sort key in the table). Unless we intend to handle filtering and pagination in the backend because of a tremendous amount of data, there's no purpose to optimising this and incurring more network lag.

Plus, all our table data is always <20 mb even if we were to make a million coupons / campaigns so its not like a frontend can't handle sorts.