Move mailer from Bamboo to Swoosh

Closes #491 (closed)

Breaking changes: Change Bamboo.SMTPAdapter to Swoosh.Adapters.SMTP and rename the server key to relay. Remove the hostname key. The MOBILIZON_SMTP_HOSTNAME environment key is not used anymore for Docker.

 config :mobilizon, Mobilizon.Web.Email.Mailer,
-   adapter: Bamboo.SMTPAdapter,
+   adapter: Swoosh.Adapters.SMTP,
-   server: "localhost",
+   relay: "localhost",
-   hostname: "localhost",
   # usually 25, 465 or 587
   port: 25,
   username: nil,
   password: nil,
   # can be `:always` or `:never`
   tls: :if_available,
   allowed_tls_versions: [:tlsv1, :"tlsv1.1", :"tlsv1.2"],
   retries: 1,
   # can be `true`
   no_mx_lookups: false,
   # can be `:always`. If your smtp relay requires authentication set it to `:always`.
   auth: :if_available