Part of EMG-2825.
We need to save web push subscription info on the keyserver inside the device_token. Unfortunately it's too long for the current VARCHAR(255) so this diff changes that to a mediumtext. Additionaly indices had to be modified because of the the type change. Now we need to specify a prefix length that they apply to. I've tested it and it seems like registration info is 400-500 bytes so I've set it to 512.