Add is_YYYY_donor and latest donation to wmf_donor

Uses wmf_donor to track fiscal years in which a contact has donated,
as well as date and original/USD amounts of latest contribution.

A civicrm_post hook keeps the data up to date when contributions are
added or edited.

To be used for WMF LYBUNT report and silverpop export.

Depends on CiviCRM change I4abdc4e608abf57b1443916f620b413791fc3d02
to allow falsy db defaults for custom fields

Deploy: stop QC, drush updatedb; run wmf_donor_initialize.sql

Change-Id: I906d4661d5e90ea2559caa05d3da1818bfdee12a