[Email Feature] add relay smtp server configuration

[changchaishi]

feat(smtp-relay): add relay smtp server configuration

Types of changes

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• Bugfix
• New feature
• Refactoring
• Breaking change (any change that would cause existing functionality to not work as expected)
• Documentation Update
• Other (please describe)

Description

目前 dev@pycon.tw 的寄信流程處於過渡期，目前希望透過 *@python.tw 取代 dev@pycon.tw 來寄信。
當中，Google Workspace (python.tw) 的架構需要在 server (staging vm + prod vm) 架設 SMTP server 來 route email message 至 google relay server。因此增加了一個 docker service 來達成此目的。

Steps to Test This Pull Request

Steps to reproduce the behavior （只適用於 production environment）:
前置作業：

• workspace 系統管理員依照 Google Admin Console 設定 輸入
• Allowed Senders - Only addresses in my domains
• Authentication - IP list (輸入 prod vm + staging vm 的 public IP)
• TLS checked

Server 端修改

1. .env 需要新增 ALLOWED_SENDER_DOMAINS=python.tw
2. 修改 .env 的 EMAIL_URL 至 smtp://mailserver:587 (local service 會轉傳，不是直接透過 google smtp 了)
3. docker-compose pull
4. docker-compose up

Expected behavior

• 可以使用任意 @python.tw address 當作 sender 發信。

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.57%. Comparing base (dfabd13) to head (5a0a65c).
⚠️ Report is 202 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1167   +/-   ##
=======================================
  Coverage   70.57%   70.57%           
=======================================
  Files          85       85           
  Lines        3392     3392           
=======================================
  Hits         2394     2394           
  Misses        998      998           

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