Fully expand traces

[gebner]

When posting issues to Zulip you often want to copy the whole trace, right now the only way to get that is to either 1) manually expand all the subtraces, or 2) run Lean from the command-line.

I'm not sure what the best UI would be. Maybe a button to expand everything (where?). I don't think the "copy to clipboard" button should do it by itself (because it's super slow in general).

See also #271.