Use HamFrequency for dealing with frequencies.

Summary: Use the `HamFrequency` class introduced in D74.

Test Plan: None yet, but I'm not going to land this until after D74 lands.

Reviewers: jcarter

Reviewed By: jcarter

Maniphest Tasks: T65

Differential Revision: https://phabricator.edgesysdesign.com/D75