Allow admins to fork repos even when creation limits are exhausted (#3277)

This is a continuation of #2728, with a test case added.

Fixes #2633.

I kept @zareck 's commit as is, because I believe it is correct. We can't move the check to `owner.CanForkRepo()`, because `owner` is the future owner of the forked repo, and may be an organization. We need to check the admin permission of the `doer`, like in the case of repository creation.

I verified that the test fails without the `ForkRepository` change, and passes with it.

Co-authored-by: Cassio Zareck <cassiomilczareck@gmail.com>
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/3277
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: Gergely Nagy <forgejo@gergo.csillger.hu>
Co-committed-by: Gergely Nagy <forgejo@gergo.csillger.hu>
(cherry picked from commit ea4071ca9f)

services/repository/fork.go

@@ -54,7 +54,7 @@ type ForkRepoOptions struct {
 // ForkRepository forks a repository
 func ForkRepository(ctx context.Context, doer, owner *user_model.User, opts ForkRepoOptions) (*repo_model.Repository, error) {
 	// Fork is prohibited, if user has reached maximum limit of repositories
-	if !owner.CanForkRepo() {
+	if !doer.IsAdmin && !owner.CanForkRepo() {
 		return nil, repo_model.ErrReachLimitOfRepo{
 			Limit: owner.MaxRepoCreation,
 		}