Move display methods to the decorator for User and Account

1. Create UserDecorator and add these methods: full_name, display_name, informal_name. They must have the same behaviour that they had in the User model before this issue/task. Add tests.
2. Remove those methods from the model. Remove the tests accordingly.
3. Create AccountDecorator that returns the display_name of its admin_user. The method must be called admin_user_display_name. When admin_user is nil, this method must return nil. Add tests.
4. Change all calls in the code to retrieve full_name, display_name, informal_name or the display_name of the admin_user to use the decorators.

Dev Notes

For this PR, the .decorate is done inside the views and helpers. To be refactored in next PRs/issues to be done from the controllers.
But in this issue, it is already clean for mailers, CSV exporter and Analytics UserTracking.