Only update Review.timestamp when saving that field
Only update Review.timestamp when saving that field

A previous patch (/r/9068/) updated `Review.save` to pass along all
keyword arguments, such as `update_fields`. However, we were still
always updating the timestamp field on the model, even if it wasn't
being saved to the database. This lead to inconsistencies between fields
on objects and fields in the database, causing a test failure. We now
only update the timestamp field specifically when we are updating all
fields or include `timestamp` in `update_fields` so that this does not
happen.

Testing Done:
With this patch applied, all unit tests pass.

Reviewed at https://reviews.reviewboard.org/r/9070/

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