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/utils/validators.ts

@@ -0,0 +1,31 @@
+/* eslint-disable @typescript-eslint/ban-types */
+import {
+  registerDecorator,
+  ValidationArguments,
+  ValidationOptions,
+} from "class-validator";
+
+export function CannotUseWithout(
+  property: string,
+  validationOptions?: ValidationOptions
+) {
+  return function (object: Object, propertyName: string) {
+    registerDecorator({
+      name: "cannotUseWithout",
+      target: object.constructor,
+      propertyName: propertyName,
+      constraints: [property],
+      options: validationOptions,
+      validator: {
+        validate<T>(value: T, args: ValidationArguments) {
+          const object = (args.object as unknown) as T;
+          const required = args.constraints[0] as string;
+          return object[required] !== undefined;
+        },
+        defaultMessage(args: ValidationArguments) {
+          return `${propertyName} cannot be used without ${args.constraints[0]}.`;
+        },
+      },
+    });
+  };
+}