Admin: reduce the fields included in user exports

[acdha]

See #624

[coveralls]

Coverage decreased (-0.02%) to 71.722% when pulling 561add7 on update-admin-user-export-columns into 9c2aeb7 on master.

[rstorey]

I'm fine with these changes, but I'd like @elainekamlley to confirm the fields they want - did they want to remove some of the extraneous fields, or only include the username and email address?

[elainekamlley]

This is good. The only concern I had is that the export included password hashes.