Deprecate generateProperties

Summary:
deprecate generateProperties() from class
DifferentialRevisionDetailRenderer. Custom fields now provides a much
more powerful version of generateProperties().

Depends on D814.

Test Plan:
implemented facebook task field with custom field and
verified it worked.

Reviewers: epriestley, tuomaspelkonen

Reviewed By: epriestley

CC: aran, jungejason, epriestley

Differential Revision: 826