Ability to group competitions by challenge.

[scorreia]

At some point, if we want to be able to distinguish competitions that are part of the FSGT challenge from other competitions internal to each gym, we need a way to group competitions together.
This will help later to filter by challenge (say "Challenge IDF Saison 2023", "Nanterre CarnaGrimpe"...).
This will also help to count the statistics of the challenge (best gym, best climbers of the season,...)

[scorreia]

we need therefore a new object called challenge that aggregate a list of competitions.
The challenge can be created at any time.
Hence, we must be able to add a competition or remove a competition from it until the challenge is closed.

From the challenge page we must be able to see/generate the results (in draft if the challenge is not closed).