Allow mapping/reassignment from placeholder-users to project/group access token bot users

Proposal

Related to #537445 (closed)

Similar to the above issue, it would be beneficial to be able to reassign contributions for placeholder users to Project/Group Access Token bot users. There can be cases where an imported project would create placeholder users based on project/group access token bot users.

Currently, these users cannot be deleted, and it resolves in placeholder users being leftover, which can be confusing when viewing users on the instance as an Administrator or when viewing the top-level group as an Owner.