Allow searching by path when using the invite widget
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### Problem to solve
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When adding approvers to merge request approval rules or attempting to invite a user/group to a project, our current search implementation doesn't return results when a group's full path is entered. This means users are forced to search for the relevant group by using only the name of the group. Since we do not currently restrict eligible approvers by their permissions (see #2048), searching only by the name of the group may return results from other groups that using the same name, making the list of eligible approvers unnecessarily long.
For example, if I'd like to add `my-group/my-subgroup` as an approver, I'd have to search for `my-subgroup` and scroll through the list of returned results until I found my specific group. The following screenshot shows this behavior:

### Intended users
Anyone working with other groups on GitLab.
### User experience goal
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The user should be able to quickly find and add the specific group they're looking for without having to filter through irrelevant groups that lay outside the scope of their project.
### Proposal
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Allow users to search for their intended group by using the full path to the group as the search criteria (e.g. `/my-group/my-subgroup`).
### Further details
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### Permissions and Security
<!-- What permissions are required to perform the described actions? Are they consistent with the existing permissions as documented for users, groups, and projects as appropriate? Is the proposed behavior consistent between the UI, API, and other access methods (e.g. email replies)?-->
### Documentation
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### Availability & Testing
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What risks does this change pose to our availability? How might it affect the quality of the product? What additional test coverage or changes to tests will be needed? Will it require cross-browser testing?
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* Unit test changes
* Integration test changes
* End-to-end test change
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### What does success look like, and how can we measure that?
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### What is the type of buyer?
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### Is this a cross-stage feature?
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### Links / references
issue