Site admin could create repos even MAX_CREATION_LIMIT=0 (#4645)

* site admin could create repos even MAX_CREATION_LIMIT=0

* Optimize if structure

models/repo.go

@@ -1411,7 +1411,7 @@ func createRepository(e *xorm.Session, doer, u *User, repo *Repository) (err err
 
 // CreateRepository creates a repository for the user/organization u.
 func CreateRepository(doer, u *User, opts CreateRepoOptions) (_ *Repository, err error) {
-	if !u.CanCreateRepo() {
+	if !doer.IsAdmin && !u.CanCreateRepo() {
 		return nil, ErrReachLimitOfRepo{u.MaxRepoCreation}
 	}