R1:98aae51c3dd8

Fix an issue with mentions in transactions

Ref T6367. Fixes T8415.

Maniphest filters transactions too early. This happens automatically later. Remove the code.

Transactions should be filtered per-user. If a transaction is hidden for some users, we shouldn't mail them. Move the filtering logic to be per-user.

Stack:

```
[Thu Jun 04 05:51:05.371061 2015] [:error] [pid 25111] [client 127.0.0.1:56497] [2015-06-04 05:51:05] EXCEPTION: (Exception) Transaction ("PHID-XACT-TASK-x4jlylat47s6ttr", of type "core…