Refine/tidy CfP states

[SamLR]

We now have 6 types of Proposal in the CfP. These are often managed semi-independently with different portions being opened/closed as necessary (e.g. lightning talks will often open very close to the event, after the main CfP has closed and close in a staggered manner during the event).

This should fix the inconsistencies raised in #1161 and add an 'OPEN' feature flag in addition to the 'CLOSED'

... they should probably all accept datetimes so things can be primed rather than manually set...