replaced old symlink logic with tested middlewear based on fs.realpath

app/coffee/SymlinkCheckerMiddlewear.coffee

@@ -0,0 +1,17 @@
+Path = require("path")
+fs = require("fs")
+Settings = require("settings-sharelatex")
+logger = require("logger-sharelatex")
+
+
+module.exports = (req, res, next)->
+	basePath = Path.resolve("#{Settings.path.compilesDir}/#{req.params.project_id}")
+	requestedFsPath = Path.normalize("#{basePath}/#{req.params[0]}")
+	fs.realpath requestedFsPath, (err, realFsPath)->
+		if err?
+			return res.send(500)
+		else if requestedFsPath != realFsPath
+			logger.warn requestedFsPath:requestedFsPath, realFsPath:realFsPath, path: req.params[0], project_id: req.params.project_id, "trying to access a different file (symlink), aborting"
+			return res.send(404)
+		else
+			return next()