Release restrictions

@Taloth @kayone This adds the backend logic for processing release restrictions (required/ignored).

While the UI hasn't been implemented:
I was going to go with the simple text field with `&:` or `|:` to denote and/or, but opted to go for a tabular design, with a dropdown/toggle to choose and/or, defaulting to or. This control would be hidden unless advanced settings are shown, I think that will make it easier all around.
I think we should do `restrictions.None()` and return `Accept` right away.