Mark users as logged in on signup and some more tasks

Currently users need to log in again after successful signup which is not a very good thing. Moreover, file uploading should be safeguarded and restricted to logged in users only. So, here are some of the tasks we gotta do:

• Automatic log in on signup.
• Check for existence of user because old cookies can validate sessions for users that don't even exist.
• Make login required for file uploading.