Edited Feature: Migrations: Consider filtering out ASP.NET Membership context when resolving single context for Enable-Migrations [1145]

If the user has the default ASP.NET Membership context in their project and one other context we should consider having Enable-Migrations just default to their other context rather than requiring them to specify which context.

This came up in a documentation review when the ASP.NET team shows that they have to always specify -ContextType when calling Enable-Migrations even when the user has just added a single context.