Prior to this change, when importing grades from .csv (or another
method) and you choose to map the grades to a 'New grade item', you get
the following error message:
```
"Mapping collision detected - two fields maps to the same grade item
new."
```
A workaround existed that required users to import one column at a time,
which was cumbersome.
This change fixes importing multiple new grade items and adds a behat
test.
Co-authored-by: hehe009 <[email protected]>
Signed-off-by: Daniel Ziegenberg <[email protected]>
