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>

modules/repository/collaborator.go

@@ -16,6 +16,14 @@ import (
 )
 
 func AddCollaborator(ctx context.Context, repo *repo_model.Repository, u *user_model.User) error {
+	if err := repo.LoadOwner(ctx); err != nil {
+		return err
+	}
+
+	if user_model.IsUserBlockedBy(ctx, u, repo.OwnerID) || user_model.IsUserBlockedBy(ctx, repo.Owner, u.ID) {
+		return user_model.ErrBlockedUser
+	}
+
 	return db.WithTx(ctx, func(ctx context.Context) error {
 		has, err := db.Exist[repo_model.Collaboration](ctx, builder.Eq{
 			"repo_id": repo.ID,