Sync Saved Scenario Users

[louispt1]

Description

See Issue #1702. This fix has been implemented in 3 commits so that the first 2 commits can be safely cherry-picked to stable. Alongside the MyETM changes, this should resolve the issues with user syncing, however two other minor changes are needed on stable in the old scenario updater to fully enable the changes:

• Add :saved_scenario_users to the .except call (around line 52 of the old updater)
• Call @scenario.sync_users_from_params(saved_scenario_users) after the
  copy_preset_roles block (around line 59)
  This represents a divergence between stable and pro, necessary because of the changes to the scenario updater.

Implementation details

Because MyETM calls: PUT /api/v3/scenarios/:id in the saved scenario callbacks, the ScenarioUpdater handles the request and therefore needs to handle the saved_scenario_users param, otherwise it will silently be dropped.

Testing

See testing details in the MyETM PR

Related Issues

See this MyETM Issue
Goes with this MyETM PR

Closes #1702