Load autosave bad state when the last save isn't a .ck file

[terryzfeng]

Steps to reproduce:

• start a new project
• upload a plain text file
• view the plain text file in editor
• refresh the page
• last saved code is the plain text file
• (now there is now default .ck file loaded)

this can potentially break webchuck export because there needs to be at least .ck file

possible fix: if autosave isn't a .ck file, start a new project with untitled.ck