Fix indentation issue.

Convert remaining tabs in edited files to spaces.
Fix PHP code with PHP Coding Standards Fixer

Fix code in app directory with php-cs-fixer.
Add configuration for php-cs-fixer tool.
Add information on how to use php-cs-fixer to CONTRIBUTING.md
Merge branch 'dev'

Conflicts:
	app/controllers/CommentsController.php
	app/controllers/FeedController.php
	app/controllers/SnippetsController.php