Allow selection of FS persistor manager. Only require the chosen persistor manager.

services/filestore/app/coffee/PersistorManager.coffee

@@ -1,6 +1,5 @@
 settings = require("settings-sharelatex")
 logger = require("logger-sharelatex")
-S3PersistorManager = require("./S3PersistorManager")
 
 # assume s3 if none specified
 settings.filestoreBackend ||= "s3"
@@ -9,6 +8,8 @@ settings.filestoreBackend ||= "s3"
 logger.log backend:settings.filestoreBackend, "Loading backend"
 module.exports = switch settings.filestoreBackend
 	when "s3"
-		S3PersistorManager
+		require("./S3PersistorManager")
+	when "fs"
+		require("./FSPersistorManager")
 	else
 		throw new Error( "Unknown filestore backend: #{settings.filestoreBackend}" )

services/filestore/config/settings.development.coffee

@@ -7,6 +7,7 @@ module.exports =
 	# which persistor to use for file storage
 	# current options are:
 	# "s3" - Amazon S3
+	# "fs" - local filesystem
 	# if no persistor is chosen, s3 will be used by default
 	filestoreBackend: "s3"