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/logging/tracing.ts

@@ -1,10 +1,11 @@
 import { init, tracer, addTags, markAsError } from "@theo.gravity/datadog-apm";
+import env from "@server/env";
 
 export * as APM from "@theo.gravity/datadog-apm";
 
 // If the DataDog agent is installed and the DD_API_KEY environment variable is
 // in the environment then we can safely attempt to start the DD tracer
-if (process.env.DD_API_KEY) {
+if (env.DD_API_KEY) {
   init(
     {
       // SOURCE_COMMIT is used by Docker Hub
@@ -13,7 +14,7 @@ if (process.env.DD_API_KEY) {
       service: process.env.DD_SERVICE || "outline",
     },
     {
-      useMock: process.env.NODE_ENV === "test",
+      useMock: env.ENVIRONMENT === "test",
     }
   );
 }