Test for non-executable files with execute bit set.

* Install a build check to avoid regressions.
Flake8 already checks for NL at EOF (W292).
Remove execute flag from non-executable files.

* Targeted file extensions
  cpp, css, eot, html, jpg, js, json, less, map, md, png, scss, ttf, txt,
  yml, woff
* Other files:
  LICENSE, README