Hackathon submission state buttons are visually confusing

The admin hackathon show page contains buttons to transition the state of a hackathon submission (Approve, Reject, and Hold for Review). It's a bit confusing because all the buttons appear at once (similar to https://github.com/hackclub/hackathons-backend/issues/401)


We should hide certain buttons depending on the current state. For example, `approve` -> `approve` should not be a valid state transition; thus, we should hide the `approve` button if the current state is approved.

```mermaid
---
title: Hackathon Submission State Diagram
---
stateDiagram-v2
[*] --> Pending
Pending --> Approved
Pending --> Rejected
Approved --> Pending: Hold for Review
Rejected --> Pending: Hold for Review
```