Closes #201: Branch cloning

[jnovinger]

Closes: #201

NOTE:

This is a "rebased" version of #202, where all commits (except for merging in main and feature) were cherry-picked to a new branch that started on feature at e9f7ca4.

Implements the ability to pull changes from one branch into another by replaying the change records from the source branch onto the destination.

• Add related_branch ForeignKey on BranchEvent model
• Implements a pull() method on the Branch model to effect the replaying of changes
• Introduces a view & REST API endpoint for "pull" operations (consistent with sync & merge operations)
• Introduces the pre_pull & post_pull signals
• Add pull_validators configuration parameter

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