Add endpoint for updating rosetta stone statements

This MR adds an endpoint for updating rosetta stone statements. This MR also...

• ...renames the latest_version property to latest_version_id
• ...replaces the is_latest_version property with a version_id property
• ...adds a test for RosettaStoneStatementTemplateCreateValidator

The endpoint path, http method and response code are up for discussion. Reasoning behind the current choices:

• Endpoint path /api/rosetta-stone/statements/{id}/versions was used because we could use /api/rosetta-stone/statements/{id} to update the modifiability and visibility of all statement versions at once
• POST instead of PUT or PATCH because we do not really modify any data and POST is not idempotent
• Response code is 201 created, as we technically create a version resource