FE - User Contribution Mapping - filter for placeholder users in the admin view
### Problem Currently, in the Admin view for instance members, placeholder users are shown under the `Users` tab and don't have their own `Placeholders` tab. This was noticed in our [test session](https://gitlab.com/gitlab-org/foundations/import-and-integrate/discussions/-/issues/112#note_2077660851) and just recently was reported as confusing behavior from a customer. Update: It has been decided to only add a filter that shows the list of placeholders. See [this comment](https://gitlab.com/gitlab-org/gitlab/-/issues/521974#note_2407312470) ![image](/uploads/6f2e11fc65f46a161f27883d51747be2/image.png) ### Proposal (Updated) Add filter in the UI so that Admin can filter for type=placeholder users in the filter bar. ~~Add filter in the UI so that Admin can filter placeholders out. By default the filter should be on, meaning by default the placeholders are filtered out.~~
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