Allow instances to be marked as hidden similar to be communities

[Ategon]

Requirements

• [X] Is this a feature request? For questions or discussions use https://lemmy.ml/c/lemmy_support
• [X] Did you check to see if this issue already exists?
• [X] Is this only a feature request? Do not put multiple feature requests in one issue.
• [X] Is this a backend issue? Use the lemmy-ui repo for UI / frontend issues.

Is your proposal related to a problem?

Currently if you want to mark all communities in an instance as hidden you need to do it for every single community

Describe the solution you'd like.

Adding in an endpoint such as api/v3/site/hide (or a better endpoint if another one makes more sense) for sites to be hidden. This hides all communities from that instance similar to the community hiding but just does it for all of them

Describe alternatives you've considered.

Theres adding this into federation to add it as a different federation level but I feel this specific thing is a bit different from federation as a whole since you still get everything and people can still see the content if they subscribe

Additional context

Use case: hidding the lemmit.online communities. People in our instance want to interact with them but we dont want them flooding the feed and hiding them one at a time tends to be a ton of thing to hide (+ more pop up as people make the instance fetch them)