Add multi-user support to scenarios (backend)

[thomas-qah]

What?

This PR adds multi-user scenario functionality to ETEngine.

Why?

Some of our clients would like to be able to share scenarios and collaborate on them together with other users. A way to do so is by being able to link multiple users to one scenario and have the ability to give them different roles. This PR does just that.

How?

Backend

• Addition of a new model:
  • ScenarioUser: This model serves as the coupling link between Scenarios and Users and symbolizes the role that a user has in relation to a given scenario. For the model to work a scenario_users table was also added. This table contains all said information.
• Addition of 'roles' to the User model:

  ROLES = {
  1 => :scenario_viewer,
  2 => :scenario_collaborator,
  3 => :scenario_owner
  }

• Renaming Scenario.owner to Scenario.users, which is now Array of Users and no longer one User.
• API::ScenarioUsersController: This is the controller handling API requests towards /api/v3/scenarios/:id/users/. It handles requests for:
  • GET /: Returns all ScenarioUsers for a given Scenario
  • POST /: Handles adding multiple ScenarioUsers in one go. Expects an array in the form of: [{user_id:, user_email:, role:}, {...}, ...]. Additionally, an invitation_args object can be included in each user object to make ETEngine send out an invitation mail to the user that was just added to the scenario. For this, it uses the new ScenarioInvitationMailer described below.
  • PUT /: Handles updating multiple ScenarioUsers in one go. Expects an array in the form of: [{id: / user_id: / user_email:, role:}, {...}, ...]
  • DESTROY /: Handles destroying multiple SavedScenarioUsers in one go. Expects an array in the form of: [{id: / user_id: / user_email:}, {...}, ...]
• A new ScenarioInvitationMailer was added to send out invitation mails to both existing and new users (for which the email address was not found). The contents of the mails themselves can found in app/views/scenario_invitation_mailer/,
• Changes to CanCan abilities to make sure a viewer can only read, a collaborator can only change the scenario but not manage its users, and an owner can do everything to a Scenario,
• Various changes all over the codebase, i.e. to the ScenarioSerializer, and YearInterpolator to keep working properly despite changes ownership (i.e. the change from owner to users described above).

Closes #1375 .

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