Fix 0011481: Don't show bug group actions that can't be used

Currently the bug action group dropdown list on view_all_bug_page.php
shows (almost) every available option, even if the current user isn't
authorised to use those options.

This patch implements checking to see which options are available to the
user for the issues which are currently shown on view_all_bug_page.php.
Options are only displayed in the dropdown list if the user is able to
use the option on at least one of the bugs displayed.

Additionally, the logic behind when selection checkboxes are shown
alongside a bug has been improved. Checkboxes won't display next to bugs
that the user cannot perform group actions on.