Added .jshintrc

- Also fixes a few JSHint warnings in files app
- Added "global" comment on top of files app to suppress warning and
  also inform devs about what globals are use
Fixed file name validation unit test + added newline

- fixed file name validation unit test
- added "\n" as forbidden character in isFileNameValid()
Merge pull request #7011 from owncloud/jshint

Added .jshintrc