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>

templates/user/auth/signin_inner.tmpl

@@ -31,6 +31,8 @@
 			</div>
 			{{end}}
 
+			{{template "user/auth/captcha" .}}
+
 			<div class="inline field">
 				<label></label>
 				<button class="ui green button">