refactor(config): use secret files in dev mode

- Refactor `config/dev.exs` to use `config/dev.secret.exs` instead of
  fetching the SMTP information from the environment and using hardcoded
  database credentials. This is useful when different developers have
  different database credentials and they use different SMTP adapters
- Add a sample `dev.secret.exs` in `config/dev.secret.sample.exs`. Also
  gitignore the origin `dev.secret.exs` file
- Add instructions on how to setup the config file for development in
  `README.md`
- Add `Plug.Swoosh.MailboxPreview` plug on "/mailbox" route in
  development mode for easier development with e-mails

Signed-off-by: Akshit Garg <garg.akshit@gmail.com>

.gitignore

@@ -35,4 +35,7 @@ npm-debug.log
 /priv/native/
 .idea
 .env
-.envrc
+.envrc
+
+# Secret config files
+config/*.secret.exs