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](4ed884f3-e06a-4862-afd3-3b8aa2488dc6)


![grafik](ae6d4981-f252-4f50-a429-04f0f9f1cdf1)


![grafik](ca153599-5b0f-4b4a-90fe-18bdfd6f0b6b)

---------

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

routers/web/repo/setting/setting.go

@@ -5,6 +5,7 @@
 package setting
 
 import (
+	"errors"
 	"fmt"
 	"net/http"
 	"strconv"
@@ -782,6 +783,8 @@ func SettingsPost(ctx *context.Context) {
 				ctx.RenderWithErr(ctx.Tr("repo.settings.new_owner_has_same_repo"), tplSettingsOptions, nil)
 			} else if models.IsErrRepoTransferInProgress(err) {
 				ctx.RenderWithErr(ctx.Tr("repo.settings.transfer_in_progress"), tplSettingsOptions, nil)
+			} else if errors.Is(err, user_model.ErrBlockedUser) {
+				ctx.RenderWithErr(ctx.Tr("repo.settings.transfer.blocked_user"), tplSettingsOptions, nil)
 			} else {
 				ctx.ServerError("TransferOwnership", err)
 			}