Allows for better testability.
Until now, many online textures were retrieved via the default texture store, which causes them to never be removed from GPU memory. It also has a performance overhead due to mipmap generation (which will be avoided via ppy/osu-framework#1885.
They were previously breaking the two-class-per-file rule.
- Also remove unnecessary fallback from absolute URL helper
- Add a show user action to link handling
- Add missing JSON fields to response model - Add missing enum value