move .imap files from user directory into their own directory.

In my /sbbs/data/user/0001 directory I've got 2094 .imap files. While I'm not concerned about the number, I think it'd be nicer, neater to have these in their own 0001.imap directory.

While not a priority, it would be good for the next major update so moving of the existing files can be done in the update.js file.