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>

custom/conf/app.example.ini

@@ -759,6 +759,9 @@ ROUTER = console
 ;; Enable captcha validation for registration
 ;ENABLE_CAPTCHA = false
 ;;
+;; Enable this to require captcha validation for login
+;REQUIRE_CAPTCHA_FOR_LOGIN = false
+;;
 ;; Type of captcha you want to use. Options: image, recaptcha, hcaptcha, mcaptcha.
 ;CAPTCHA_TYPE = image
 ;;