Make sure only one category of a taxonomy can be assigned to a [rec/action/user] if taxonomy.allow_multiple is false

[tmfrnz]

currently it appears that the server allows making multiple connections even if taxonomy.allow_multiple is false

[tmfrnz]

• [x] actually instead of returning an error when a previous association exists, the server should simply remove the previous association and replace it with the new one

• [x] now there may be cases where the client is requesting the removal of an existing association at the same time as requesting the creation of new one, which -when implementing the above- may result in the request to remove a link that was already removed by the server. thus to prevent returning an error the request to delete a non-existing association should yield a positive response

[tmfrnz]

duplicate of https://github.com/impactoss/impactoss-server/issues/286