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>
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