A column needs to be added to every CheckUser result table so that it can store the associated user agent string in the `cu_useragent` table.
The column will be named `agent_id` with the column prefix for the table appended to the name. For example, for the `cu_changes` table the column name would be `cuc_agent_id`. The column will be a bigint to be the same type as the `cuua_id` column in the `cu_useragent` table.
======Acceptance criteria
* [] Create the column in each of the `cu_changes`, `cu_log_event`, and `cu_private_event` tables.
======Proposed schema
```
> describe cu_changes;
+----------------+---------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+----------------+---------------------+------+-----+---------+----------------+
....
| cuc_agent_id | bigint(20) unsigned | NO | | 0 | |
+-- -------------+---------------------+------+-----+---------+----------------+
> describe cu_log_event;
+----------------+---------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+----------------+---------------------+------+-----+---------+----------------+
....
| cule_agent_id | bigint(20) unsigned | NO | | 0 | |
+-- -------------+---------------------+------+-----+---------+----------------+
> describe cu_private_event;
+----------------+---------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+----------------+---------------------+------+-----+---------+----------------+
....
| cupe_agent_id | bigint(20) unsigned | NO | | 0 | |
+-- -------------+---------------------+------+-----+---------+----------------+
```