Add through field and remove through annotation code

With this change, the throughproperty is turned from an annotation to a model field.

The effect of this is to show the latest share author for all relevant streams instead of a few selected streams. The rationale is that since shares are public, there should be no visibility issue in showing the last share author for all streams with public content.

An other effect is the author re-sharing their own content will also be shown.

This work was initiated to allow for homogeneous content ordering across streams, especially in the context of the new UI (WIP). More details on https://codeberg.org/socialhome/socialhome-ui.

TODO: once this request has been reviewed and is approved, fix some tests that are broken (which was expected).