This commit operates at three levels of abstraction:
1. We want to prevent user history rows from being unbounded in size.
This commit adds rails validations to limit the sizes of columns on
user_histories,
2. However, we don't want to prevent certain actions from being
completed if these columns are too long. In those cases, we truncate
the values that are given and store the truncated versions,
3. For endpoints that perform staff actions, we can further control
what is permitted by explicitly validating the params that are given
before attempting the action,
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