Refactor to improve discoverability of DEA NATS messages
Limit service instance names to 50 characters

[Finishes #50667131]
App placement prefers DEAs with more available memory

Also extract private EligibleDeaAdvertisementFilter class

[Closes #50684677]
Move gateway callbacks from service binding to service instance.
decouple Sequel validations from DB constraints

This concept previously existed, however any DB constrained attributes also required validations.
Remove default values in ServiceBinding
Expose interface for creating object from model controller
Supports creating service bindings for provided service instances.
Limit service_instance name column to 50 chars [#50667131]
Update README.md

Service Gateways do not advertise services over NATS
Update README.md

Service gateways do not respond to provision requests over NATS
Update README.md

bin/console is broken. Dropping the mention until it's fixed
All DEA staging requests are now async so we are dropping support for synchronous staging.
Fix a regression when refactoring: sending nil service binding gateway name instead of service_instance name.
Refactor the service binding to staging request
Refactor start app message to use ServiceBindingPresenter.
Support VCAP_SERVICES for provided services
Return empty hash if binding options nil [#50701663]
Add /v2/service_plan_visibilities endpoints

These endpoints (GET, POST, DELETE) allow you to manage which
organizations have access to which private plans.

[#51577237]
Make service plan visibilities dependent on orgs and plans

Deleting organizations or service plans will delete associated service
plan visibilities

[Finishes #51577237]