Feature: Copy milestone and associated issues between projects

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Proposal

A US Government customer is interested in copying milestones and associated issues from one project to another or multiple projects. Ideally, during the copy operation, the issues would retain metadata. /copy_metadata #issue exists, however, this only copies metadata between issues within the same project.

For example:

* Project A
  - Milestone 1
    - Issue X
    - Issue Y

copy Milestone 1, Issue X, and Issue Y to Project B

Relevant info:

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