User management

[main--]

• [ ] Login (steam?)
• [ ] managing users and their demos on the server (mongo stuff)
• [ ] provide a way to select previously uploaded demos in the results/upload/everything view
• [ ] decide whether users should be allowed to download their demos (maybe a feature for pro accounts?)
• [ ] decide whether users should be allowed to delete their demos (~)

anything else?

[moritzuehling]

Do we really need this?