Deprecate 'return false' in event handlers

Fixes #2015. Wasn't sure how to handle `react-version` in _config.yml, given that this is a v0.12 milestone.
FWIW we already have these in the code:

```
monitorCodeUse('react_object_map_children');
monitorCodeUse('react_no_scroll_event');
```

I guess the call stack isn't particularly useful though in this case.