A new Mixed-feed which merges the Subscribed, Local and All feed based on probabilities customable by the user e.g. 10/40/50 or 0/20/80

[bluebbberry]

Requirements

• This is a feature request and not a bug report. Otherwise, please create a new bug report instead.
• Please check to see if this request (or a similar one) already exists.
• It's a single feature. Please don't request multiple features in one issue.

Describe the feature you'd like

When I currently browse Lemmy, I first check the All-feed, then switch to Subscribed and finally to the Local-feed. It would be nice to have all of that in one feed. The feed would simply merge the three main feeds into one by randomly picking from each feed. The probabilities of this should be customaziable, either with sliders directly in the UI, or more likely, in the instance's or user's settings.

Here is a mockup how the feature could look like:

The posts of the different feeds are marked by color, because this would enable users to easily check if the algorithm works correctly.

Additionally to the ease-of-use benefits, a Mixed-Feed would also solve a basic problem of interest on Lemmy between choosing the Local- or the All-Feed as the default. Currently, most instances I saw have the Local-Feed set as default. This is especially preferable for smaller instances because it keeps their user engagement local. If they set the default to the All-feed, their posts would almost never appear there. However, it also leads to the fact that users stay on their instance, user engagement does not flow through the threadiverse as it should and there is less cross-instance-interaction, which hurts overall discussion quality. A mixed-feed on the other hand, could combine the merits of both feeds, helping smaller instances show their content to the user while also distributing user engagement to federated instances.

The feature was already partly discussed here: https://lemmy.world/post/5157579.