Project templates: Enable/disable custom templates
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### Problem to solve
GitLab introduces custom project templates starting with [11.2](https://gitlab.com/gitlab-org/gitlab-ee/issues/6860), allowing to bootstrap a new project based on prepared configuration and boilerplate code.
Since the amount of out-of-box templates and custom templates can become large quite fast, we should implement a way to allow enabling/disabling build-in vendor templates.
### Proposal
Let's add the option for Administrators and Group Owners to enable/disable individual built-in templates in the Admin Area (instance-level) and Group settings (groupo-level).
select one group in the GitLab instance that holds GitLab projects that are offered as project templates, when creating a new project.
### Designs
#### Enable/disable templates
**Admin area → Project templates → Built-in templates**
| List | Empty | Undefined |
|------|-------|-----------|
|  |  |  |
### What does success look like, and how can we measure that?
* If a custom template is disabled, it does not appear in the "Create new project from template" flow.
issue