Computed properties already defined in data

Steps to reproduce

1. Launch Funkwhale (develop version)
2. Except for /manage/settings, visit any page at /manage/
3. Open developer console
4. You'll find warnings of "computed properties already defined in data"

What happens?

Computed properties defined in the Ordering mixin are also defined in the components that use that mixin.

What is expected?

No warnings

Context

When using the mixin routePreferences in ui.js should be used to define those properties.