Fixes DS-293: Adds PartnerMember model and PartnerMember-User relations

This addresses several requests in DS-293:

* Creates PartnerMember model in Eloquent ORM
* Ensures that API-created members get an entry in PartnerMembers, with flOriginal = 1
* Ensures that existing members, upon API authentication, have a row in PartnerMembers, with flOriginal=0
* Ensures that calls to member info (`/users/123`) only return if a relationship exists for this user and this partner in PartnerMembers
* Sets PartnerID in the `.env` file