Project templates: Enable/disable built-in templates
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### Problem to solve
GitLab ships with built-in vendor project templates. In addition, with https://gitlab.com/gitlab-org/gitlab-ce/issues/48043 we are introducing project templates, 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 built-in vendor templates. Right now, these are
* Ruby on Rails
* Spring
* NodeJS Express
### Proposal
Let's add the option for Administrators to enable/disable the built-in templates in the Admin Area.
### Solution
_tbd_
### Earlier 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?
**For the team (internal):**
* Administrators can enable or disable built-in project templates globally via Admin Area settings
* The setting is also manageable via API for organizations that manage configuration programmatically
**For customers (measurable after the feature ships):**
* Adoption of the setting by organizations with custom template libraries (target: upward trend among self-managed Ultimate customers)
* Reduction in feedback and requests related to template confusion or accidental use of built-in templates
issue