accept key auth for all API v2 actions

API v2 does not accept key auth for all actions prior to this patch. The question is if the actions should be authorised already in the parent controllers or only in the API seeing as most other actions are already authorised in the parent controllers, hence the overwriting of `#accept_key_auth_actions`.

In any case this makes key auth work for the remaining API v2 actions.

[WP #2734](https://www.openproject.org/work_packages/2734)