contest: alllow not assigned contestant to make semifinal event wish after qualifs end_date and before PROLOGIN_WISHES_OVERRIDE_UNTIL

[lportemo]

Two Things must be done :

• Create a new BooleanField on the Edition model to allow assigned_semifinal contestant to select their assignation_event
• Create a page where the contestant can select its event.
• Add a link to the page on the chronological frieze (display it only when event selections are opened)

[lportemo]

Si personne veux prendre je prends