[Trustee Index] Use controlled vocabulary for trustee filtering

As a user, I want to be able to select from a limited list of trustee types and see any trustee that has served as that type of trustee.

[x] When I click on the dropdown for "Trustee Type", I see the following list only: [Alumni, Charter, ex Officio, Term, President, Governor]
[x] When I select, e.g., "Alumni", I will see anyone who served in this capacity, whether or not they also served in other capacities.
[x] I can successfully use this feature with Voice Over in Safari.
[x] I have checked this feature using one of the following browser extensions and confirmed that it does not add any level A or level AA issues:
- axe dev tools
- Lighthouse
- Wave

If I select "Term" under trustee type, "Livingston Tallmadge Merchant '26" is included in the results list.

pulibrary / static-tables