Make hexathons more easily switchable

[ayush-goyal]

Allow for people to view multiple hexathons at the same time similar to how registration works with the "Select event" screen. This will involve changing the backend config so that we have one row of config for each hexathon individually. Also, you'll need to switch a lot of the frontend requests to send the hexathonId similar to how registration works. This will allow us to see past expo data without need to switch the "current hexathon" toggle, and we can showcase past event expos as well.