Stop logging authorized_teacher permissions to slack

[balderdash]

This PR stops logging to slack when a teacher gets the 'authorized_teacher' permission. It also fixes the bug that logged each new permission to slack twice.

Over the last couple months, 1152 new user permissions have been created, of which 1123 were authorized teacher. Most of those were created in 5 large batches of ~200 teachers each:

mysql> SELECT permission,month(created_at),day(created_at),COUNT(0) FROM user_permissions WHERE created_at > '2017-12-1' AND permission='authorized_teacher' GROUP BY month(created_at), day(created_at);
+--------------------+-------------------+-----------------+----------+
| permission         | month(created_at) | day(created_at) | COUNT(0) |
+--------------------+-------------------+-----------------+----------+
| authorized_teacher |                 1 |               2 |        1 |
| authorized_teacher |                 1 |               3 |        4 |
| authorized_teacher |                 1 |               4 |      184 |
| authorized_teacher |                 1 |               7 |        1 |
| authorized_teacher |                 1 |               8 |        1 |
| authorized_teacher |                 1 |               9 |        1 |
| authorized_teacher |                 1 |              10 |        3 |
| authorized_teacher |                 1 |              11 |        2 |
| authorized_teacher |                 1 |              12 |      195 |
| authorized_teacher |                 1 |              16 |        1 |
| authorized_teacher |                 1 |              17 |        2 |
| authorized_teacher |                 1 |              18 |        2 |
| authorized_teacher |                 1 |              19 |        1 |
| authorized_teacher |                 1 |              20 |        5 |
| authorized_teacher |                 1 |              22 |        5 |
| authorized_teacher |                 1 |              23 |        2 |
| authorized_teacher |                 1 |              24 |      243 |
| authorized_teacher |                 1 |              25 |        2 |
| authorized_teacher |                 1 |              29 |        3 |
| authorized_teacher |                 1 |              30 |        3 |
| authorized_teacher |                12 |               2 |        1 |
| authorized_teacher |                12 |               4 |        1 |
| authorized_teacher |                12 |               6 |      259 |
| authorized_teacher |                12 |               7 |        1 |
| authorized_teacher |                12 |               8 |        1 |
| authorized_teacher |                12 |               9 |        1 |
| authorized_teacher |                12 |              12 |        1 |
| authorized_teacher |                12 |              13 |        1 |
| authorized_teacher |                12 |              15 |      193 |
| authorized_teacher |                12 |              20 |        2 |
| authorized_teacher |                12 |              26 |        1 |
+--------------------+-------------------+-----------------+----------+

These batches of new permissions end up spamming the #infra-security room, making it very difficult for casual observers to spot unexpected permission changes as they scroll by. While it's good to have a log of permissions changes (perhaps we should send authorized_teacher changes to firehose instead?), the slack room is much more useful if we actually have the time to see everything that gets logged to it.