Add support for multiple Default Forms (Site Wide)

[n7studios]

Summary

Resolves this reported issue, allowing one or more non-inline site wide forms to be defined at Kit > Settings > Default Forms (Site Wide):

Testing

• NonInlineFormCest: Updated tests to check two or more non-inline Forms are supported
• testChangeDefaultSiteWideFormsSettingAndPreviewFormLinks: Test that non-inline Forms specified at Kit > Settings > Default Forms (Site Wide) works with 1 and 2 forms.

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)