Save the URL for the doodle to a dated file, use that in the checker

[johnmckerrell]

It would be good to send out the request for cover earlier in the week, say on Wednesday, to give people more time to plan.

The cover requester needs updating to append the date that the week begins in the filename it saves the URL to and then the cover checker needs to be changed to use this (probably just reading the config and then this file rather than assuming it'll be an argument).