Migrations/Imports: Handle Re-Imports by Creating Copies

[kdmccormick]

The initial API supports two ways to handle re-imported blocks:

1. replace_existing == True: When a block is imported a second time, update it. In other words, prefer the source (course) version.
2. replace_existing == False: When a block is imported a second time, skip it. In other words, prefer the target (library) version.

However, in the UI, we want a third, different behavior: when a block is imported a second time, create it as second forked version.

This means that the API should support three ways to handle reimported blocks:

1. repeat_handling_strategy == "update"
2. repeat_handling_strategy == "skip"
3. repeat_handling_strategy == "fork" <- new

cc @cmltaWt0 - future work for us

[rpenido]

Hi @kdmccormick! This is available on the sandbox if you want to try it.

Can we mark this as done?