Redirect on bad CSRF instead of presenting bad page (#14937)

The current CSRF handler is a bit harsh with bad CSRF tokens on webpages
I think we can be a little kinder and redirect to base page with a flash error

Signed-off-by: Andrew Thornton <art27@cantab.net>

options/locale/locale_en-US.ini

@@ -100,6 +100,8 @@ never = Never
 [error]
 occurred = An error has occurred
 report_message = If you are sure this is a Gitea bug, please search for issue on <a href="https://github.com/go-gitea/gitea/issues">GitHub</a> and open new issue if necessary.
+missing_csrf = Bad Request: no CSRF token present
+invalid_csrf = Bad Request: Invalid CSRF token
 
 [startpage]
 app_desc = A painless, self-hosted Git service