Only allow multiple todos per action per object

Merged Heinrich Lee Yu requested to merge multiple-todos-per-action into master

What does this MR do and why?

Changes the behavior of the multiple_todos feature flag so that only one todo per action type is allowed per object.

An exception is made for mentions so that users can still have multiple mention todos since these link to different notes.

This reduces noise for the other todo actions where they link to the same thing.

Related to #28355 (comment 864628139)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

Edited by Heinrich Lee Yu