New Filter that only Admins can apply to projects - but available to all when browsing projects

**As a:**
- Code for Philly organizer
- Interested in helping new volunteers find projects

**I want**
- A new "Volunteers Wanted" _filterable project atribute_ created
- That is usable when anyone browses the site,
- but can only be applied to projects when an Admin chooses to do so.

**So that**
- new members can find projects that are actively interested in onboarding new volunteers
- and that Admins can verify/validate projects are in fact "ready" to receive and onboard new volunteers

Start by locating the project-browsing filter and the admin project-editing flow. Confirm how project attributes are stored and how admin permissions are checked. Done means anyone can filter for “Volunteers Wanted,” while only an Admin can apply that attribute to a project.

索引モデルが issue の本文から書いたものです。