Warn about usage of non-breaking space

Non-breaking space is often not visible in our tools, or shown as normal
space. We should use &nbsp; instead. Warn about non-breaking space like
we warn about tabs.

Change-Id: I1bbbb1f19c56d3cc1b949df151b41886a0c46042
Related-Bug: #1314498
Merge "Warn about usage of non-breaking space"