Limit mapping fields to count of available CF7 forms

[felipeelia]

Q	A
Bug fix?
New feature?
Issues addressed (#s or URLs)	#14
BC breaks?
Deprecations?
Your wordpress.org's username	@felipeelia

Description: Currently it's only possible to map 5 forms. This PR makes possible mapping all available forms.

This PR solves an immediate problem, but IMO, the select boxes aren't the ideal solution. We should have a list of all CF7 forms with a list of input fields, with names as name='cf7_mautic_settings[form_id][{$post_id}]' or something like that. What do you think? If you agree with that I can change the necessary code. The only problem would be backward compatibility, as it would change the current behavior of using a bidimensional array and array_search to find the id of given post_id.

Steps to test this PR:

1. Access the configuration page
2. Check the quantity of available forms
3. Add ou remove a form
4. Access the configuration page again
5. Recheck the quantity

As we do have just a 1:1 relation, I think we should make possible to user map all forms.