feat(#653): add --merge-primary-contacts flag for merge-contacts action

[kennsippell]

Description

When --merge-primary-contacts is present during merge-contacts:

• If both source and destination have a primary contact, the source primary contacts are deleted and all of their reports reassigned to the destination primary contact
• If either source or destination don't have a primary contact, then the source primary contact is moved normally with no reports reassigned
• If the destination has no primary contact, I considered updating the destination doc to use the source primary contact. Currently, nothing ever results in the destination contact being updated so I decided this might be overstepping.

#653

Base branch is #652

Code review items

• Readable: Concise, well named, follows the style guide, documented if necessary.
• Documented: Configuration and user documentation on cht-docs
• Tested: Unit and/or integration tests where appropriate
• Backwards compatible: Works with existing data and configuration. Any breaking changes documented in the release notes.

License

The software is provided under AGPL-3.0. Contributions to this project are accepted under the same license.

[kennsippell]

One last review for these related PRs. Thanks @sugat009!