UI: Equation Editor

[bryantrobbins]

To support the definition of new statistics from existing columns in a dataset, Baseball Workbench should have an Equation Editor.

• Users should be able to easily add references to columns from their datasource.
• Users should be able to easily add references to custom columns previously defined.
• Attempts to add references to non-existent columns should result in a client-side error.
• Users should be able to use simple mathematical operators: Add, Subtract, Multiply, Divide
• Attempts to use unsupported operators (or include any extraneous characters) should result in a client-side error.

[bryantrobbins]

Descoping this from the MVP. Will add in next round.