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/hooks/useAuthorizedSettingsConfig.ts

@@ -163,7 +163,7 @@ const useAuthorizedSettingsConfig = () => {
         name: "Slack",
         path: "/settings/integrations/slack",
         component: Slack,
-        enabled: can.update && (!!env.SLACK_KEY || isHosted),
+        enabled: can.update && (!!env.SLACK_CLIENT_ID || isHosted),
         group: t("Integrations"),
         icon: SlackIcon,
       },