Add start_date and end_date parameters to the get-assignments request

[ots22]

schedule.rkt> (get-the-schedule)
; Failed to connect to Forecast #"HTTP/1.1 422 Unprocessable Entity"

Verified Forecast id and token, and that Forecast is up.

Failing step is get-assignments (team, project and clients all work correctly):

...
schedule.rkt> (fc:get-assignments <forecast>)
; Failed to connect to Forecast #"HTTP/1.1 422 Unprocessable Entity"

[ots22]

• Wimbledon is also unable to update at the moment, although that might be unrelated
• The Forecast webapp successfully connects to https://api.forecastapp.com/assignments?end_date=2021-05-02&start_date=2021-04-05&state=active (the same endpoint as get-assignments, with some parameters)

Edit: Indeed, the reponse contains {"errors":["start_date and end_date must be present."]}