Relative path calculation on the backend

[gogiel]

Related to #171

It aims to replace normalizePath filter which:

• doesn't work well if path includes /app/
• doesn't support files in other directories, like lib/

rails_panel/rails_panel/assets/javascripts/filters.js

Lines 34 to 38 in 4204111

	filter('normalizePath', function() {
	return function(input) {
	return input.remove(/.*\/app\//);
	}
	}).

Frontend changes are backward compatible.