Add Intercom to Settings and Setup Wizard Screens

[n7studios]

Summary

Adds the Intercom help messenger to the Plugin's settings and setup wizard screens, to encourage users to seek support direct from Kit, instead of leaving a review.

The user_id isn't populated, as Identity Verification is enabled, requiring a user_hash.

Testing

• PluginIntercomCest: Test that the Intercom messenger displays on the Plugin and Wizard screens.

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)