import: Clean the management command.

[rheaparekh]

Cleans up:

• Remove the --import-into-nonempty option (we make that behavior the default). We leave the option present in argparse (and just have it print a warning if passed for now)

• Improve manage.py import to handle being passed a tarball or give a clear error message if you pass it a tarball.

• Improve the argument parsing error handling for manage.py import (currently if you forget that argument, it gets confused and tries to use the filename as a subdomain, and gives a confusing error)

I am bit unsure about the third point in issue #9550

I'll also add to my TODO list to update the docs according to this new changes (depreciation of import-into-nonempty) after 1.9 release.

[timabbott]

Merged as 66e3fc4, after a bit more cleanup and simplification. It does feel a lot nicer now :).

I ended up replacing the tarfile check with just an "is this a directory" one, since that handles other errors nicely as well. Thanks @rheaparekh!

[rheaparekh]

@timabbott Yes its definitely much better now!