Add option to limit number of submissions for a form

[AngellusMortis]

As a user, I would like to be able to limit the number of submissions made to a form so that the form does not get too many respondents.

We have had a use case to automatically close a form and stop new submissions to a form once it reaches a set limit of submissions (like a form asking for interest in a training classes, you need to stop people from submitting after you get ~30 submissions or something).

I have already made this change on an internal fork we have and can submit the PR if interested.

What I did:

• added new field to AbstractForm for max_submissions, < 0 = no max.
• added new status for "Closed", closed differs from "Draft" in that it still will display the form page will load and not return a 404.
  • If you are a non-staff user, you will get the form title and intro for a closed form as well as an error message stating the form is closed to new submissions.
  • If you are a staff user, you will still get the full form (for testing) and upon submission, you will get a error message stating the form is closed to new submissions.
  • Forms will automatically switch to "Closed" upon the first form that is submitted that reaches the max_submissions value if it is positive
• published on both the FormManager and AbstractForm now returns forms of "Published" and "Closed" forms. Logic for "Closed" forms is only enforced in the view logic

[luzfcb]

cool new feature.