classic ui: add 'executed command' to extinfo.cgi linking to command expander

fixes #3901
core: fix unable to exclude hostgroups when using nested hostgroups (Alexander Sulfrian) #2619

patch adds a dedicated reject list, which then also allows to exclude
hostgroups. thanks for the neat patch.

a testcase is added as well, it passes config verification where
previously an error was thrown.

refs #2619