Non-child group for group-level project template
### Problem to solve
<!--- What problem do we solve? -->
Ability to use group-level project templates from a group who is not a child of the current group.
### Target audience
<!--- For whom are we doing this? Include either a persona from https://design.gitlab.com/getting-started/personas or define a specific company role. e.a. "Release Manager" or "Security Analyst" -->
Developer
### Further details
<!--- Include use cases, benefits, and/or goals (contributes to our vision?) -->
To industrialize the creation of our developers' projects we would like to use the project templates. We have two types of templates, "build" templates for building application images, and "deploy" templates for setting up application execution environments.
Here is the tree structure of our gitlab :
```
├──docker
| ├── templates
| | ├── build
| | | └── projects "build" template
| | └── deploy
| | └── projects "deploy" template
| ├── environment
| | ├── production
| | | └── projects created from "deploy" template
| | └── testing
| | └── projects created from "deploy" template
├──developper team 1
| └── projects created from "build" template
└──developper team 2
└── projects created from "build" template
```
We have chosen to define the "build" templates in instance-level project. We would-like to use the group-level project template for the "deploy" template.
The problem is that our group containing the "deploy" templates projects is not a child of the environment subgroups.
### Proposal
<!--- How are we going to solve the problem? -->
Be able to define any group as group-level project template.
### Links / references
Relate to #6861
issue