Selectively load columns for differential typeahead

Summary:
Change the differential typeahead to only load columns that it needs. To do
this, I also enabled partial objects for PhabricatorUser (and made necessary
changes to support this). I also changed the functionality of Lisk's loadColumns
to either accept columns as multiple string arguments or a single array of
strings.

Test Plan:
With tokenizer.ondemand set to false, checked that the typeahead loaded and I
can type multiple people's names. Set tokenizer.ondemand to true and tried
again. In both cases, the typeahead worked.

Reviewers: epriestley

Reviewed By: epriestley

CC: jungejason, aran, epriestley, nh

Differential Revision: 990