Allowing private Push-Gateway

Description:

After change https://github.com/RocketChat/Rocket.Chat/commit/dfeebb8afcbfabf25cabc49e4fb9618f7b6859d2 it is impossible to use a private push gateway (like https://github.com/arminfelder/RocketChatMobilePushGateway) without enabling Register_Server and Cloud_Service_Agree_PrivacyTerms (see below). I understand the need to limit the usage of the provided push-gateway, but limiting people to use their own gateways is stretching the described agenda a bit far.

https://github.com/RocketChat/Rocket.Chat/blob/b913621b5718fb89ae90e5640d0fdd119978ba46/app/push/server/push.js#L73-L77

We have over 30 instances and one customer client for each plattform. Spreading the app-push-credentials over all instances would be quite painful.

Steps to reproduce:

Go to settings > push and try to enter a custom push gateway without accepting the Cloud Agreement and registering the server.

Expected behavior:

Be able to add a custom server and send http-push requests to it.

Actual behavior:

The settings GUI does not allow to set a custom server. If it is set using environment variables, Cloud_Service_Agree_PrivacyTerms and Register_Server has to be enabled to make push requests happen.

Server Setup Information:

• Version of Rocket.Chat Server: 3.6.2
• Operating System: Linux RedHat CoreOS / OpenShift
• Deployment Method: K8S
• Number of Running Instances: ~30
• DB Replicaset Oplog: true
• NodeJS Version: 12.16.1 - x64
• MongoDB Version: 3.6.3

Client Setup Information

• Operating System: Desktop, iOS, Android

Start at app/push/server/push.js lines 73-77 and reproduce the custom push gateway setup described in the issue, including the settings GUI and environment-variable paths. Verify that a custom server can be configured and receive HTTP push requests without enabling Cloud_Service_Agree_PrivacyTerms or Register_Server, then check the existing push behavior for regressions.