Move Comments option to Discussion settings

Summary

To reduce complexity (and confusion for our admins), we should group all comment-related controls in one menu.

As indicated in the Handbook page all comment-related settings are now available at Dashboard > Settings > Discussion, so we should move the opt-in checkbox for submitting the form introduced in PLANET-6465 to this location.

Requirements

• Move Comments opt-in settings from Planet 4 > Comments to Settings > Discussion.
• Completely remove the Comments section from Planet 4 menu.