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/emails/templates/CollectionNotificationEmail.tsx

@@ -1,5 +1,6 @@
 import invariant from "invariant";
 import * as React from "react";
+import env from "@server/env";
 import { Collection } from "@server/models";
 import BaseEmail from "./BaseEmail";
 import Body from "./components/Body";
@@ -54,7 +55,7 @@ ${collection.name}
 
 ${collection.user.name} ${eventName} the collection "${collection.name}"
 
-Open Collection: ${process.env.URL}${collection.url}
+Open Collection: ${env.URL}${collection.url}
 `;
   }
 
@@ -75,7 +76,7 @@ Open Collection: ${process.env.URL}${collection.url}
           </p>
           <EmptySpace height={10} />
           <p>
-            <Button href={`${process.env.URL}${collection.url}`}>
+            <Button href={`${env.URL}${collection.url}`}>
               Open Collection
             </Button>
           </p>