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.
nit: can we say "e.stopPropagation() or e.preventDefault(), as appropriate."? usually people only need one and I'd like to encourage them to use the correct one instead of both like a sledgehammer. :)