Support the transfer of groups

Description

Support the transfer of groups

As in Issue #2772 stated there should be an option to move existing groups into other groups.

This would make reorganizing large groups to a group/subgroup structure much easier and avoids moving hundreds of projects manually.

Proposal

Allow groups and subgroups to be transferred around between other groups:

• Subgroups to groups
• Top-level groups to groups
• Top-level groups to subgroups

Links / references

#2772 (closed)

Documentation blurb

(Write the start of the documentation of this feature here, include:

1. Why should someone use it; what's the underlying problem.

Transfer of of group and all of it's project to another namespace

2. How does someone use this

From the groups settings choose the new target group where the current group should reside

During implementation, this can then be copied and used as a starter for the documentation.)