signing.wont_sign.%!s(<nil>) will be displayed if the repository needs signed
commits but the user is not logged in.

This is displayed because of complicated logic in the the template repo/issue/view_content/pull.tmpl
and a shortcut in the code of routers/repo/issue.go

This PR adds a default value of notsignedin if users are not signed in, which
although our templates will not show will prevent custom templates from showing
the above.

It also fixes the template to avoid showing signing errors if the user is not
authorized to sign.

Replaces #12564
Close #12564

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