Rethink the date system and how dates are defined

[Ambient-Impact]

The current system relies on a wiki node having one date, but this will change in neurocracy/drupal-omnipedia-core#10 which will require a partial or total rethink of how dates are defined.

The solution most like the current system would be to build a list of dates from those defined in wiki node body fields, but that's complicated and has most of the same issues the current system has.

A better solution may be to define dates explicitly as their own entities, which could make keeping a list of dates easier, as we could have an entity reference to all wiki nodes that reference them. We would need to build in a good amount of validation to ensure that dates are continuous and aren't missing dates in between; it may be best to not even expose the ability to set or change individual dates, but expose just the start and end dates as configurable, with the system creating or deleting dates as needed to fill the date range in; this would also need extra care if the start or end date is changed to be shorter, as this would require rewriting wiki node content and editors should not be able to do this without a big and obvious confirm step that lists all the nodes that would be affected and what the changes would be.

See also #1