adds cleanup functionality for old devices

- device now last field 'last connection' which gets updated
  at every server connection from the respective device
- manual cleanup script in database deletes all devices which
  have never been updated, or have a timestamp older than 30 days

server/routes.py

@@ -1,6 +1,7 @@
 # Python standard libraries
 import json
 import os
+import time
 
 # Third-party libraries
 import flask
@@ -204,6 +205,7 @@ def downloader(device):
     if request_device.user_id == None: 
         return jsonify(kind="unregistered")
 
+    request_device.lastConnection=int(round(time.time()))
     request_device.connection=True
     db.session.commit()
     request_user = db.session.query(User).filter(User.id==request_device.user_id).first()