Can't create a new draft if another user has authored one

See this thread

Currently a draft is only editable by the draft author, even if it's owned by an organisation. A user in the same organisation can view the draft, but can't edit the tree in the builder view (this is also not very obvious - a message on the page to indicate that it's readonly would be nice).

The option to create a draft from a published/under-review version is also disabled if there's an existing draft (with any author).

This means that if user A has created a draft owned by a particular organisation, user B can see the published versions and the draft, but can't edit the draft, and also can't create a new one. Since the codelist is organisation-owned, there should be a way for a user to make a new version without having to contact the existing draft author and have them either delete or put their draft under review.

Options:

1. Make drafts editable by all users in the organisation, not just the author
2. Allow users to create a new draft if they haven't already authored one - this means there could be multiple drafts for a single codelist)

Start with the linked Slack thread and trace the draft edit and creation flows described in the issue; no files or tests are named. First resolve which organization behavior is intended, then verify that an organization user can make the permitted change and that the read-only state is clear when editing is not allowed.