FE: Placeholder user should not appear in dropdowns

What does this MR do and why?

Dropdowns often include the author of the issue, MR, epic, etc. This causes the placeholder user to appear at the top of the dropdown list that appears when assigning something to a user. We would like the placeholder not to appear as assignees for the following issuables:

• Issues
• MRs

The previously merged MRs below updated the UserCore graphql type to include a "type" property, and updated the 'assignees' queries to include the type field for each fetched user, which this current MR uses to filter out users with type placeholder.

• !173568 (merged) added the field 'type' to the UserCore graphql type.
• !175972 (merged) makes type available to frontend

This current MR then filters out users with type placeholder in the sidebar-assignees-widget component.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After
Migrated Issue assignee list	Migrated Issue assignee list
Migrated Merge request assignees list	Migrated Merge request assignees list
VideoBefore.mov	AfterVideo.mov

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Create an issue and then migrate the group in which you created the issue
   • You do not need the feature flags enabled for user contribution mapping anymore, but they are: bulk_import_importer_user_mapping and importer_user_mapping
   • choose one of your groups at http://127.0.0.1:3000/dashboard/groups (or create a new group)
   • create a new issue in one of the projects under your chosen group. Create an MR as well
   • assign the issue and MR to yourself.
   • go through the process of migrating your chosen group into another top level group. The steps are at the link below. Make sure to 'import with projects' when you're on the page where you choose which group to import https://docs.gitlab.com/ee/user/group/import/direct_transfer_migrations.html
2. After the transfer is done, go to the new subgroup and the issue you previously created, which was migrated.
3. The issue and MR should have "Placeholder" as the author under "created by"
4. Open the assignees dropdown, and you should not see any assignee with the word 'placeholder' in the name.

Related to #505564 (closed)