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: D922: asdf
subject-prefix: [Differential]
vary-subject-prefix: [Request, 100 lines]
...
RECIPIENTS
! duck (duck)
    - This mail has tags which control which users receive it, and this recipient has not elected to receive mail with any of the tags on this message (Settings > Email Preferences).

BODY
epriestley requested code review of "asdf".
...
```

Verified update mail is marked undeliverable:

```
$ ./bin/mail show-outbound --id 6646
...
Message: Message has no valid recipients: all To/Cc are disabled, invalid, or configured not to receive this mail.

PARAMETERS
...
mailtags: ["differential-updated"]
...
subject: D922: asdf
subject-prefix: [Differential]
vary-subject-prefix: [Updated, 100 lines]
...
RECIPIENTS
! duck (duck)
    - This mail has tags which control which users receive it, and this recipient has not elected to receive mail with any of the tags on this message (Settings > Email Preferences).

BODY
epriestley updated the revision "asdf".
...
```

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3030

Differential Revision: https://secure.phabricator.com/D6518