Revert "Use print_form_button() instead of print_small_button()"

Trying to detach an Issue from a Changeset always results in
APPLICATION ERROR 2800 - Invalid form security token, even when using
a valid token.

This is because print_form_button() currently does not work with action
URLs generated by plugin_page(), see [1].

So we revert to original behavior of using print_small_button() (as it
was before commit 787eaa84), but keeping
the single form security token implemented in commit
634277d2.

This change can be reverted again, once the upstream issue has been
fixed.

Issue #372

[1]: https://mantisbt.org/bugs/view.php?id=28533