Add new condition builder

[lukeholder]

Add condition builder for logical conditions.

Use in:

• Discounts
• Sales
• Tax
• Shipping

Will fix #181
Should fix #467

Look into https://dabernathy89.github.io/vue-query-builder/ ?

Condition and effects should be separated for discounts and sales so effected products/variants can be different from conditions.

[lukeholder]

Instead of sales types or discount types, we should move to plugins providing condition objects we can use in a rules builder.

Conditions would really be like a fieldtype, but offer a way to match (true or false) an element

A condition object would provide:

inputHtml() like fields for the conditions params to be entered.
a list of elements the condition is compatible with
a match() method to match the condition with the element.

[lukeholder]

Need to also be able to generate all combinations of sale prices for all matching purchasables for each sale, so that the hasSales param on the product/variant/purchasable query can be a simple join.