[Marketplace] Don't trust a passed in shop

[queso]

We are passing in a shop to the packages pub. https://github.com/reactioncommerce/reaction-core/blob/development/server/publications.coffee#L89

certainly make no sense as is, should as least be ` shop = shop || ReactionCore.getCurrentShop(@)`.    I think this is more a hint of the functionality needed for the marketplace(multi-shop)..  which might publications might need a shop param (or something like this https://github.com/DispatchMe/Meteor-restricted-find)

[queso]

@aaronjudd can you explain the concept of a multi-shop more? Is this something supported now, or planned for in the future?

[aaronjudd]

yes, see #357.

We've begun implementing parts of this. and there are other community members developing there own versions of this, and I feel like we should try at least add to the schema/model/publications handling, even if we don't build any UI for this right now.