Use print_form_button() to display "Detach" buttons in view page

[dregad]

Current use of print_small_button() generates a complex GET URL showing the form security token. It would be better to use print_form_button() so the token is handled as hidden form parameter.

Moreover multiple tokens are generated (one per issue), which is not efficient,

[dregad]

Initial attempt to fix this (see 787eaa8) actually does not work - the Detach button always throws APPLICATION ERROR 2800 - Invalid form security token.

The problem persists even after fixing the problem with the invalid token (see 634277d).

This is because print_form_button() currently does not work with action URLs generated by plugin_page(), see MantisBT issue #28533.

This change has to be reverted for now and put on hold until the upstream problem has been fixed, probably in MantisBT 2.26.0 (PR mantisbt/mantisbt#1753).