Filter out user repositories and forks when searching code
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### Problem to solve
It is difficult to search for code when:
1. You need a global search (not just one specific group)
1. User repositories and/or forks of upstream repositories clutter the results
### Intended users
Developers
### Further details
When using a self-hosted GitLab instance, code is often distributed across a number of different GitLab groups. If the engineer's workflow involves forking the repository, searching code proves impractical, because user forks are mixed in with the rest of the results. It would be useful to be able to filter results to remove forks and/or user repositories when searching code. This may also apply to other search patterns (gitlab-ce#30771).
An example I find myself regularly encountering is searching across all repositories for upstream repositories that have a .gitlab-ci.yml that contains a particular runner tag.
### Proposal
Add Syntax to allow the ability to filter out:
* Forks
* User repositories
*
### Permissions and Security
I don't think this has permissions or security implications.
### Documentation
I'm not sure if I'm supposed to do this? If so let me know.
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### Testing
This could require changes to the elastic search schema, which would require testing.
### What does success look like, and how can we measure that?
I can easily only search the contents of "groups" in GitLab, and I can easily search all of GitLab, but ignore forks.
### Links / references
Relates to gitlab-ce#30771
issue