Add ability to create blackout dates for hostings

[jtschulz]

This should be accessible in users/{user_id}/hostings/new and /hostings/edit views. We don't have a design on this currently. I think it makes sense to use a modal with a calendar, then a list of blackout dates below the "message for visitors" input with buttons to update or delete them and a button to create a new blackout date range.

First step is to create some wireframes. Ideas/ help with that is appreciated.