Show connected projects for a specific (project) runner
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### Release notes
placeholder for release notes
### Affected view(s)
{project-name}/~/settings/ci_cd/runners/{runner_id}/edit
{project-name}/~/settings/ci_cd/runners/{runner_id}
### Problem to solve
- A user may have a runner that is added directly to many individual projects. However, as a user, you have no idea which other projects may be using the runner. To get this information, you will need access to the database and RAILS console. Most users do not have this access.
- What's worse - you may find that "your" runner is available for sharing in another project. You would like to lock it - but for that, you need to go to any projects where the runner is associated. However, the current GitLab UI doesn't give you any clues as to which projects to check.
- The user interaction on the two affected views also need to be improved. If the user chooses to edit the specific (project) runner by selecting the `edit` icon, changes a value in the form, and then clicks `save changes`, the end result is a re-direct to the details view. And now, there is no easy way for the user to navigate back to the edit form.
### Proposal
- One option, pending design by the UX team, is to add a `show connected projects` link/button.
- On click, display a pop up showing the connected projects.
- The connected projects data can be retrieved from the DB using `Ci::Runner.find(runner_id).projects.map(&:full_path)`
### Documentation
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