Fix Event Cancellations

- Declared variable names will override column names in MySQL. There was
a declared variable name of event_id which broke the functionality of:
"SELECT FROM `event_cancellations` WHERE `event_id` = event_id"
The fix was to prefix the table name to the column:
"SELECT FROM `event_cancellations` WHERE
`event_cancellations`.`event_id` = event_id"