Implement a menu to add xfn gates.

[jrobbins]

This should resolve a request from the API Owners and save me from doing manual work. Sometimes a feature is created using the PSA feature type, but then it turns out to have a bigger impact than expected, so it needs cross-functional review and an intent-to-ship thread.

In this PR:

• Add a "..." menu to the stage accordion sections with a "Add cross-functional gates" item when appropriate.
• Add an API call to csClient and main.py for the new action.
• Add an API handler to add the needed gates.

[jrobbins]

I still need to add new unit tests, but you can take a look now.

[jrobbins]

OK, ready for review.

[dlaliberte]

I can't really review the Python code, but the JavaScript seems fine, though I wonder about the use of confirm and event triggering.