[SERV-460] Determine item access mode from visibility field in CSV

[markmatney]

The --access-mode flag is now an optional override.

Here's the new help output:

$ ./import-items.py --help
Usage: import-items.py [OPTIONS] HAUTH_BASE_URL INPUT_CSV...

  Import items into Hauth.

  Instructs the Hauth instance at HAUTH_BASE_URL to register the access modes
  of all items in each INPUT_CSV.

  Unless --access-mode is supplied, the access mode of each item is determined
  from the Visibility field of its corresponding INPUT_CSV as follows:

  Visibility      Access mode
  ----------      -----------
  open            OPEN
  ucla            TIERED
  private         TIERED
  sinai           ALL_OR_NOTHING

  Exits with zero status only if all items are imported successfully.

Options:
  -m, --access-mode [OPEN|TIERED|ALL_OR_NOTHING]
                                  The access mode to use for all items in each
                                  INPUT_CSV (instead of the Visibility field
                                  value).
  -k, --api-key TEXT              The API key for accessing the admin API.
                                  [required]
  --help                          Show this message and exit.