Reader Comments: Track when notification setting is toggled

[dvdchr]

Refs #17363 (comment)
Depends on #17340

This adds a new event, reader_toggle_comment_notifications, which tracks whenever the comments notification setting is toggled. In addition, these properties are added:

• blog_id: The site ID of the blog that the user enabled the notifications on.
• notifications_enabled: true if the notifications is enabled, or false otherwise.

To test:

Make sure the "Follow Conversation via Notifications" flag is enabled. You should see 🔵 Tracked: reader_toggle_comment_notifications when:

• Tapping "Follow" > "Enable" notice action.
• Tapping the "Undo" notice action.
• Toggling the switch for the in-app notifications sheet.

Regression Notes

1. Potential unintended areas of impact
   n/a. Feature is hidden behind a flag.

2. What I did to test those areas of impact (or what existing automated tests I relied on)
   n/a. Feature is hidden behind a flag.

3. What automated tests I added (or what prevented me from doing so)
   n/a. Feature is hidden behind a flag.

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

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You can trigger an installable build for these changes by visiting CircleCI here.

[peril-wordpress-mobile]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[ScoutHarris]

Just one suggestion. Otherwise !