Make React editor indent guide work like it did in the old editor

Fixes #2367

* The indent level of empty lines is the *max* of the nearest non empty
  line, rather than favoring the level of the line below.

* An extra wrap guide is no longer rendered for empty lines

I didn't port the specs over because we already had good coverage at the
model level. It just needed to be updated for the preferred behavior.
Merge pull request #2858 from atom/ns-react-fix-wrap-guide

Make React editor indent guide work like it did in the old editor