Bug template: collapsible section for traceback

I find it nice when someone files an issue, and the super-long stack trace is collapsed. But I never remember how to do it. We could add this to the bug template.

```
<details>
<summary>Traceback</summary>

FILL IN YOUR STACK TRACE HERE

</details>
```

becomes:

<details>
<summary>Traceback</summary>

FILL IN YOUR STACK TRACE HERE

</details>