Add support for the import of unstructured gradebooks and a note about its End of Life

GRBK-1221 removed the ability to import unstrcuctured gradebooks. This is the long term goal.... for the short term we will need to continue to support them

So, this ticket should

1) accompany GRBK-1221 in deployment
2) create a new file format 'Gradebook without structure'
3) add summary text if the file format when #2 above is selected to inform users that it will a) not be fully supported and b) be removed from GB2 in a future version.