Add user blocking (#29028)

Fixes #17453

This PR adds the abbility to block a user from a personal account or
organization to restrict how the blocked user can interact with the
blocker. The docs explain what's the consequence of blocking a user.

Screenshots:


![grafik](https://github.com/go-gitea/gitea/assets/1666336/4ed884f3-e06a-4862-afd3-3b8aa2488dc6)


![grafik](https://github.com/go-gitea/gitea/assets/1666336/ae6d4981-f252-4f50-a429-04f0f9f1cdf1)


![grafik](https://github.com/go-gitea/gitea/assets/1666336/ca153599-5b0f-4b4a-90fe-18bdfd6f0b6b)

---------

Co-authored-by: Lauris BH <lauris@nix.lv>

routers/api/v1/repo/transfer.go

@@ -4,6 +4,7 @@
 package repo
 
 import (
+	"errors"
 	"fmt"
 	"net/http"
 
@@ -117,7 +118,11 @@ func Transfer(ctx *context.APIContext) {
 			return
 		}
 
-		ctx.InternalServerError(err)
+		if errors.Is(err, user_model.ErrBlockedUser) {
+			ctx.Error(http.StatusForbidden, "BlockedUser", err)
+		} else {
+			ctx.InternalServerError(err)
+		}
 		return
 	}