Remove the project dependency when reordering WorkItems

The problem

The original discussion started from the reorder API, but after deeper analysis, we found that actually there's a project dependency on the reordering algorithm.

Original discussion

The following discussion from !196269 (merged) should be addressed:

• @nicolasdular started a discussion: (+1 comment)

  blocking question Since that change would impact the API for the Frontend

  @kassio

  1. Do we want to support group-level work items as well in the future?
  2. Has the frontend access to the namespace_id ?

  If the answer is yes to both: can we instead use the namespace_id?

Proposed solution

Remove the project dependency when reordering WorkItems, specially the dependency on Project#self_or_root_group_ids.