Display corps birthday

Add a cake emoji (🎂) at the end of a corps' name in CorpsDisplay if the current date is their corps-birthday. Meaning, the date of their first rehearsal or first gig (whichever is earliest).

This date is already calculated when building the text in the component (in "Gick med i corpset..."), so preferably this value should be reused in both places. Even MORE preferably the calculation should be moved out to its own function in the corps/stats router.

To keep the router function pure either the component or router API could take `currentDate` as an argument and use that for checking if the current date is their corps-birthday, rather than using `new Date()` directly in the code, as that would make the function non-pure.

Inspecciona CorpsDisplay y el router corps/stats, empezando por el cálculo de fecha existente que se usa para el texto “Gick med i corpset...”. Reutiliza o extrae ese cálculo y ten en cuenta currentDate para que la lógica siga siendo pura. La tarea estará terminada cuando aparezca un 🎂 después del nombre del corps únicamente en el cumpleaños del corps, basándose en la fecha más antigua de un primer ensayo o de un primer concierto.

Escrito por el modelo de indexación a partir del texto del issue.