Add search filter to user multiselect for position form

[Snorre98]

Implement the functinality created in this PR in the "create position form", specificaly for searching users in the multiselect.

The userdata is fetched here, and passed as prop to the form.

You might have to create a new API call in api.ts

The exepcted outcome of solving this issue is that only users which match the search value of the multiselect is fethed from backend. I belive it would be appropriate that no users are displayed (available as options) when the search value is empty.