Add user_id to action_worker

[rrenkert]

To have an association from the action_worker to a user, a new field user_id should be added to the action_worker. The field should always contain the request user id.

Adding the field needs a migration. Since action_workers are used temporarily we can simply delete all old action_workers. @peb-adr before we merge this migration we should talk about the impact.

[rrenkert]

Talked to @peb-adr: We already use the concept of a user with the id -1 and want to use -1 for old action_workers as user_id. So in the migration all action_workers are kept with the user_id -1.

[peb-adr]

We already use the concept of a user with the id -1

To be more specific, user_id -1 is set for positions that are created as a result of an internal action.