Add ability to have webhooks triggered by events in a sub-group

Description

If you have several layers of groups and sub-groups, it can be tedious to add webhooks to every sub-group and those group's sub-groups.

Proposal

It'd be nice if there was a way to add a "cascasding" or "recursive" webhook to say, my.gitlab.instance/Some-Group which will then also get triggered for, my.gitlab.instance/Some-Group/Some-Apps-Subgroup/Some-App

Documentation blurb

When adding a webhook to a group, you can also make it trigger for projects under this group, and it's sub-groups' projects. To add webhook that gets triggered by a group and it's sub-groups projects, you just have to tick the box "Recurse Subgroups" and it will be changed to apply against this group, and every sub-group of this group.