dashboard/app: shorted asset links

Asset links are extremely long, and overflow 80 cols that we diligently tried to adhere to:
```
kernel image: https://storage.googleapis.com/syzbot-assets/13d7f9c6c13e/bzImage-3609fa95.xz
```

This should be something like:
```
kernel image: https://syzbot.org/a/13d7f9c6c13e/bzImage-3609fa95.xz
```

Also the level of indirection may be useful, if we ever migrate somewhere else.