Add a welsh_editor trait
Add a welsh_editor trait

This creates a user with the `welsh_editor` permission.

This is part of the changes required to implement the new Welsh editors
signon permissions.

Trello card: https://trello.com/c/Hq2qgwiI/2155-8-give-permission-in-publisher-for-welsh-teams-in-departments-to-update-only-welsh-content

alphagov/publisher,alphagov/publisher,alphagov/publisher