Menu timestamps

[elliotdavies]

The canteen website is updated every Monday morning. This means that if you ask for Monday's menu on a Sunday, you'll actually get the menu from six days earlier.

The timestamp currently included in the core server's output is the "parsed at" date/time, so a new field will need to be added for the date the menu pertains to. Then the Slack/Alexa integrations can make use of this in their outputs.