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

app.json

@@ -102,11 +102,11 @@
       "value": "openid profile email",
       "required": false
     },
-    "SLACK_KEY": {
+    "SLACK_CLIENT_ID": {
       "description": "See https://api.slack.com/apps to create a new Slack app. You must configure at least one of Slack or Google to control login.",
       "required": false
     },
-    "SLACK_SECRET": {
+    "SLACK_CLIENT_SECRET": {
       "description": "Your Slack client secret - d2dc414f9953226bad0a356cXXXXYYYY",
       "required": false
     },