Legacy-formatted birthdates are still being written to user database

Presumably by a web UI new user registration, resulting in update.js warnings:

Updating users birthdate field:
Updating birthdate format of ligaciputra login (#57)

Ideally, the JS User object would auto convert legacy birthdates to the YYYYMMDD format upon writing.

assigned to @rswindell

closed with commit 0bdcdf09

mentioned in commit 0bdcdf09

mentioned in commit 932d26a7