Show nice stack trace in call view

- [ ] Remove the `traceback` database field from `Call`. We're going to start storing more complex data than text now, so use the `data` field instead.
- [ ] Format the traceback in a similar way to Flask/werkzeug: in particular being able to click on a line to view context around it, and click again to collapse.
- [ ] Include the traceback for all calls, but for calls that didn't end with an exception, the traceback should be completely collapsed/hidden to begin with, so the user can optionally view it but it's usually out of the way.
- [ ] For calls with exceptions, highlight the stack frame in the traceback that corresponds to the call being viewed.

In a future PR we can add local variables and links to other traced calls.