Add endpoint for getting leaderboard metadata by invite code

[Eldemarkki]

We need an endpoint for getting leaderboard metadata by an invite code for Testaustime/testaustime-frontend#270

The metadata should include at least:

• Leaderboard name
• Member count

The endpoint could for example be GET /leaderboards/invite-code/ttlic_xyz123. The endpoint should not require authentication, because the links should be shareable to anonymous users and we should show them the metadata and prompt them to register to Testaustime in order to join the leaderboard.