Environments manual actions should only be available when `playable`
When an environments has manual actions that are not playable, when clicked they send the user to a 404 page. In order to avoid this situation we could render them as disabled.
We can send this information in the payload of the API response, with a flag making sure we do not induce the user in error.
More information here:
- https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7015#note_18558490
- https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7015#note_18560303
- https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7015#note_18560712
//cc @grzesiek