Metabase: Support calculated columns in questions

[agilliland]

Goal: The goal of this feature is to decrease the number of important queries in the system that need to be expressed in SQL.

User Stories:

• A user should be able to ask questions that are expressed using add, subtract, multiply, or divide operators
• A user should be able to calculate net profit from a cost and expense field

[mazameli]

Here's a design prototype of where we're headed with this:
https://invis.io/PK6LYRCM3