refactor: update mailer service to stop using postmark (#38)

* refactor: update mailer service to stop using postmark

* chore: remove postmark as a dep and move out postmark code

* chore: remove postmark variables from .env.example

* chore: add formal errors for mailer initialization errors

* chore: add and update jsdoc comments in mailer service methods

packages/hoppscotch-backend/nest-cli.json

@@ -1,5 +1,11 @@
 {
   "$schema": "https://json.schemastore.org/nest-cli",
   "collection": "@nestjs/schematics",
-  "sourceRoot": "src"
+  "sourceRoot": "src",
+  "compilerOptions": {
+    "assets": [
+      "**/*.hbs"
+    ],
+    "watchAssets": true
+  }
 }