Allow multiple todos behind a feature flag

Merged Heinrich Lee Yu requested to merge 28355-multiple-todos-backend into master

What does this MR do?

Allow multiple todos per object when the feature flag is enabled.

The feature flag is per-user.

There are some UX concerns / bugs mentioned in #28355 (comment 447072597) but this is meant to be an experiment so we can see if this is a good change then we can address those.

Related to #28355

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

Availability and Testing

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

  • Label as security and @ mention @gitlab-com/gl-security/appsec
  • The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
  • Security reports checked/validated by a reviewer from the AppSec team
Edited by Heinrich Lee Yu