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>

services/repository/fork.go

@@ -53,6 +53,14 @@ type ForkRepoOptions struct {
 
 // ForkRepository forks a repository
 func ForkRepository(ctx context.Context, doer, owner *user_model.User, opts ForkRepoOptions) (*repo_model.Repository, error) {
+	if err := opts.BaseRepo.LoadOwner(ctx); err != nil {
+		return nil, err
+	}
+
+	if user_model.IsUserBlockedBy(ctx, doer, opts.BaseRepo.Owner.ID) {
+		return nil, user_model.ErrBlockedUser
+	}
+
 	// Fork is prohibited, if user has reached maximum limit of repositories
 	if !owner.CanForkRepo() {
 		return nil, repo_model.ErrReachLimitOfRepo{