Standardize handling of effective dates

[showerst]

Related to openstates/issues#219

I did a quick check and we're using multiple different extras keys for effective dates, and also capturing the dates but not adding them as an extra in a few states.

I propose we:

1. Standardize on two extras, 'date_effective' and 'date_effective_str'
2. 'date_effective' is for states where we can do a nice Y-m-d , 'date_effective_str' for MN which makes different parts of the legislation effective on different dates, so we can just dump the whole instruction string in.

I can probably knock this work out later this week, if it's the route we want to go down.

@jessemortenson - ping so you don't miss this

[jamesturk]

I believe I haven't thought as much about this as either of you two, so mostly going to defer to you.

FWIW, I'm fine with standardizing the extras, I'd also be up for adding something to the core schema but maybe there's more discussion needed first.

[showerst]

Closing this, as I plan to move this to an OSEP so we can handle it properly in core.