DRY up selectize users helpers

(cherry picked from commit eb38ddc6c761d4920b35499690fd5681f0ea5385)

Checklist

• I have read the Contribution & Best practices Guide.
• My branch is up-to-date with the upstream master branch.
• The tests pass locally with my changes.

Short description of what this resolves/which issues does this fix?:

• selectize... helpers had duplicate code.

Changes proposed in this pull request:

• DRY up selectize... helpers.