Current expo should be auto-selected based on time

[andrewsosa]

Right now the current expo is selected based on a current bool field. Instead, the current expo should be determinable if the current time is between a "start" and "end" time defined by the expo object.

We can change the current bool field to be a currentOverride to manually set the active expo even if we aren't between the times.

https://github.com/HackFSU/hackfsu_com/blob/dev/api/api/models/judging_expo.py