Unify field naming related to publication date

[maarcingebala]

There is naming inconsistency in the fields related to publication date and availability:
- Product.available_on + Product.is_published
- Page.available_on + Page.is_visible
- Collection.published_date + Collection.is_published (will be merged soon, #3369)

Also fields is_visible/is_published are problematic since they can be set to True but if the future publication date is set, an item won't be available publicly.

What I would propose to do about it:

• introduce publication_date = models.DateTimeField() field in all of the above-mentioned models (currently we have DateField but I think some may want to specify exact time as well)
• rename is_published/is_visible fields to is_draft - I think that what those fields do is telling if as an admin I've already finished editing an object or I still have something to add, but I'd like to hear some feedback about this one. Let's consistently use is_published for this kind of fields.

[patrys]

Why I like is_published over is_draft is the way it looks when used in the API/client:

query Q {
  products(isPublished: true) {
    edges {
      node {
        ...
      }
    }
  }
}

[maarcingebala]

We could have isPublished in the API and also as a helper method in the model, but it should be a property taking into account publication_date and is_draft underneath.