[FEATURE]: Add multi-league support for contests

[CloudEllie]

Problem / opportunity:

Currently, contests are added to contests.csv with a single value to indicate what chain they're on: EVM or Cosmos. But some past contests have told us that they're really both (EVM and Cosmos). This seems likely to become more of an issue as we add more chains.

Sponsors have asked if we can note on the website that their contest falls into both categories, but we haven’t had a way to do this.

Desired outcome

It would be preferable if we could:

• list multiple chains where appropriate;
• consider how to display contests that are "tagged" with multiple chains (e.g. on https://code4rena.com/contests/)
• integrate this into any UX improvements we make to our Reports section.

Solution

Add multi-league support to contests.csv so the C4 website can display them on the appropriate page/contest list.