fix for project creation: group projects don't count towards user's project limit

fix for project creation: group projects don't count towards user's project limit

I modified method check_limit in model project to not produce an error because of project limit reached when the project will be created in a group namespace.
fixes #6126

In addition it would also make sense to show the create project button on the dashboard in the case when the user has no project limit left but owns groups (and therefore can create projects in these groups). I left it out to keep the change as small as possible.