Updates to recurrent course editions

• Periods are linked to specific dates, but the projects should be recurrent (every Q1). Instead, link the periods to a specific start and end month (integer) only.
• There should be an option to make a non-recurrent proposal. This should be selectable on the proposal page in combination with a specific (already existing) period. This proposal will be linked to the actual edition only, not to the recurring one.
• It is not super clear that the proposal will be recurring, instead, there should be a checkbox "Recurring/repeated project", checked by default for recurrent course editions. Base behaviour is to recur, other behaviour is the above (specific edition, only one period can be selected and only if its course edition exists (CE can be updated in the form or hidden if that is easier)).
  • Other option is to use a flag on the project instead whether it is recurring or not.