R1:cf255cde3d20

Fix "request" and "update" mail preferences for Differential

Summary:
Fixes T3030. T1977 attempted to fix this but either didn't work (I think this is the case) or was broken later. We don't send `DifferentialCommentMail` on a create or update; we send `DifferentialReviewRequestMail`.

Also update the details to be more clear.

Test Plan:
Verified review request mail is marked undeliverable:

```
$ ./bin/mail show-outbound --id 6644
...
PARAMETERS
...
mailtags: ["differential-review-request"]
...
subject…