Extension:CentralNotice/cn known devices table

Schema summary
mysql> describe cn_known_devices; +---++--+-+-++ +---++--+-+-++ +---++--+-+-++
 * Field            | Type           | Null | Key | Default | Extra          |
 * dev_id           | int(11)        | NO   | PRI | NULL    | auto_increment |
 * dev_name         | varbinary(255) | NO   | MUL | NULL    |                |
 * dev_display_label | varbinary(255) | NO  |     | NULL    |                |