Feature/default pbc start date

Per [story 64870566](https://www.pivotaltracker.com/story/show/64870566).  Note that testing requires the task queue removal in order to work, so [PR 981](https://github.com/reciprocity/ggrc-core/pull/981) is merged with this and should be reviewed and merged first.

Allows date columns to take a default value, which is applied when a blank string is given; this feature is used for the start date column, to set it to the result of `datetime.today()`.