Implement many-to-many relationships without mapping join table

[ajcvickers]

This issue tracks work on many-to-many relationships in EF Core. Subscribe to this issue to get updates when progress is made on many-to-many relationships. This issue is locked for now to reduce noise. Please use issue #1368 for general discussion on many-to-many relationships.

[divega]

Current plan for 3.0 is to implement skip-level navigation properties as a stretch goal. If property bags (#9914) also make it into 3.0, enabling a seamless experience for many-to-many could become easier.