Adds a make target and a database for a shared channels node (#28238)

This PR allows to run a different Mattermost instance in port :8066
using the `make run-node` target. This instance reuses the frontend
files compiled for the main development instance, has its own database
and local mode enabled through environment variables, and for the rest
of the configuration simply takes whatever properties are set in the
`config.json` file.

This target is specially useful to run a Shared Channels development
environment, compiling first the frontend and then running `make
run-server` and `make run-node` side by side, later connecting both
and testing Shared Channels features.

server/build/docker/postgres_node_database.sql

@@ -0,0 +1,2 @@
+CREATE DATABASE mattermost_node_test;
+GRANT ALL PRIVILEGES ON DATABASE mattermost_node_test TO mmuser;