Add user mapping membership table and model

Luke Duncalferequested to merge
477845-migrations into master

What does this MR do and why?

This MR adds migrations and a model for import user mapping of memberships #477845 (closed).

For more context on this change, see the next MR !163077 (merged) which uses the model within services.

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Related to #477845 (closed)

Edited by Luke Duncalfe

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