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>

lib/ketbin_web/router.ex

@@ -59,7 +59,9 @@ defmodule KetbinWeb.Router do
 
     scope "/" do
       pipe_through :browser
+
       live_dashboard "/dashboard", metrics: KetbinWeb.Telemetry
+      forward "/mailbox", Plug.Swoosh.MailboxPreview
     end
   end