Add Follow Bill Functionality to Testimony Detail Page

[Mephistic]

Problem

The Testimony Detail page (e.g. https://maple-dev.vercel.app/testimony/N9n7LiycmnSqfT4Q2d7g7/1) currently has an option in its sidebar to follow the bill the testimony was written for - but this button currently does nothing.

The Follow/Unfollow bill logic is all already implemented on the backend - we just want to add that functionality to the "Follow Bill" button on the Testimony Detail page.

Success Criteria

• When the current user is not already following the relevant bill, the "Follow" button on the Testimony Detail page should read "Follow [Bill]" and clicking it should successfully follow the bill
• When the current user is already following the relevant bill, the "Follow" button on the Testimony Detail page should read "Unfollow [Bill]" and clicking it should successfully unfolllow the bill