[Email Feature] add relay smtp server configuration

[iknowright]

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

Types of changes

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• [ ] Bugfix
• [x] 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 :white_check_mark:

Project coverage is 70.57%. Comparing base (dfabd13) to head (5a0a65c). Report is 2 commits behind head on master.

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Additional details and impacted files

```diff @@ 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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