Add option to enable CAPTCHA validation for login (#21638)

Enable this to require captcha validation for user login. You also must
enable `ENABLE_CAPTCHA`.

Summary:
- Consolidate CAPTCHA template
- add CAPTCHA handle and context
- add `REQUIRE_CAPTCHA_FOR_LOGIN` config and docs
- Consolidate CAPTCHA set-up and verification code 

Partially resolved #6049 

Signed-off-by: Xinyu Zhou <i@sourcehut.net>
Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: Andrew Thornton <art27@cantab.net>

docs/content/doc/advanced/config-cheat-sheet.en-us.md

@@ -634,6 +634,7 @@ Certain queues have defaults that override the defaults set in `[queue]` (this o
 - `ENABLE_REVERSE_PROXY_FULL_NAME`: **false**: Enable this to allow to auto-registration with a
    provided full name for the user.
 - `ENABLE_CAPTCHA`: **false**: Enable this to use captcha validation for registration.
+- `REQUIRE_CAPTCHA_FOR_LOGIN`: **false**: Enable this to require captcha validation for login. You also must enable `ENABLE_CAPTCHA`.
 - `REQUIRE_EXTERNAL_REGISTRATION_CAPTCHA`: **false**: Enable this to force captcha validation
    even for External Accounts (i.e. GitHub, OpenID Connect, etc). You also must enable `ENABLE_CAPTCHA`.
 - `CAPTCHA_TYPE`: **image**: \[image, recaptcha, hcaptcha, mcaptcha\]

docs/content/doc/advanced/config-cheat-sheet.zh-cn.md

@@ -145,7 +145,8 @@ menu:
 - `ENABLE_NOTIFY_MAIL`: 是否发送工单创建等提醒邮件，需要 `Mailer` 被激活。
 - `ENABLE_REVERSE_PROXY_AUTHENTICATION`: 允许反向代理认证，更多细节见：https://github.com/gogits/gogs/issues/165
 - `ENABLE_REVERSE_PROXY_AUTO_REGISTRATION`: 允许通过反向认证做自动注册。
-- `ENABLE_CAPTCHA`: 注册时使用图片验证码。
+- `ENABLE_CAPTCHA`: **false**: 注册时使用图片验证码。
+- `REQUIRE_CAPTCHA_FOR_LOGIN`: **false**: 登录时需要图片验证码。需要同时开启 `ENABLE_CAPTCHA`。
 
 ### Service - Expore (`service.explore`)