R1:50e084dcda9c

Add a rough `bin/mail volume` command for showing mail volume

Summary: Ref T7013. This might help us understand the problem better.

Test Plan:
```
$ ./bin/mail volume
+==============+============+
| User | Unfiltered |
+==============+============+
| admin | 136 |
| dog | 31 |
| epriestley | 24 |
| ducksey | 18 |
| saurus | 8 |
| example-list | 7 |
| squeakybirdo | 3 |
| nnn | 3 |
| facebooker…