[Model Relation] hasAndBelongsToMany

Description / Steps to reproduce / Feature proposal

Originated from https://github.com/strongloop/loopback-next/issues/2043

This issue is created specific for hasAndBelongsToMany because the original issue contains other types of model relation.

A few comments from #2043 that are related to hasAndBelongsToMany.

• @bajtos gave some info and starting point on creating a new relation type: https://github.com/strongloop/loopback-next/issues/2043#issuecomment-441176508
• @RaphaelDrai posted a design doc on hasAndBelongsToMany: https://github.com/strongloop/loopback-next/issues/2043#issuecomment-447065436
  • I believe @bajtos suggested to make the design docs in .md or in gist for easier review (?)

cc @RaphaelDrai @TomerSalton @codejamninja - Since you've expressed interests to implement this model relation type, I'm assigning it to you. Please let me know if i'm wrong and i can remove you from the assignees list. Also, if there's anything I can help facilitating, please let me know.