Add 'Buying Role' field

[joshuarestivo]

Extend the Frappe Contact field with a 'Buying Role' field. This should exist as a Link to a CRM Buying Role doctype with the following default values:

Blocker
Budget Holder
Champion
Decision Maker
End user
Influencer
Technical Evaluator
Procurement
Gatekeeper
User Advocate

This list should be manageable through the frontend CRM Settings facility (similar to all other lists exposed through the frontend).