Generate unique strings for screenshots

[justinsacbibit]

Screenshot URLs currently use auto-incrementing IDs, e.g. https://trucksu.com/ss/2. This means that essentially all screenshots uploaded are easily accessible by anyone.

Instead, we should use a unique string for each screenshot. However, we have to decide what happens with existing screenshots - if we change them to use the unique string, then the existing links to them will be dead, and the user will also not know how to access them.

A couple of potential solutions to the above problem:

1. give a user access to their screenshots on the website
2. don't generate unique strings for existing screenshots, just keep them accessible through their IDs

Right now, I think that a good idea would be to go with the second option listed above. Then, we eventually implement the first option, and then convert existing screenshots to use unique strings.