chore: Centralize env parsing, validation, defaults, and deprecation notices (#3487)

* chore: Centralize env parsing, defaults, deprecation

* wip

* test

* test

* tsc

* docs, more validation

* fix: Allow empty REDIS_URL (defaults to localhost)

* test

* fix: SLACK_MESSAGE_ACTIONS not bool

* fix: Add SMTP port validation

server/models/decorators/Deprecated.ts

@@ -0,0 +1,14 @@
+/* eslint-disable @typescript-eslint/ban-types */
+
+const Deprecated = (message?: string) => (
+  target: Object,
+  propertyKey: string
+) => {
+  if (process.env[propertyKey]) {
+    console.warn(
+      `The environment variable ${propertyKey} is deprecated and will be removed in a future release. ${message}`
+    );
+  }
+};
+
+export default Deprecated;