Server error when converting a BNF codelists in some situations

Why are we doing this?

There are situations where attempting to convert a BNF codelist to DM+D results in a 500 server error. If an identical dmd codelist already exists the code should display a banner to the end user with a link to the dmd file. However the code that attempts to get the link fails if there are multiple identical slugs (which can happen as they're only unique to a user or an org).

How will we know when it's done?

The conversion no longer returns an error but instead displays the banner linking back to the existing dmd codelist

What are we doing?

This line fails if there are multiple identical slugs:

https://github.com/opensafely-core/opencodelists/blob/877690d4b59d09a93b89841433e16a3d12751ebc/codelists/views/version_dmd_convert.py#L22

Instead we should scope it to the user or org, similar (but not identical) to this code here:

https://github.com/opensafely-core/opencodelists/blob/877690d4b59d09a93b89841433e16a3d12751ebc/codelists/actions.py#L760-L763

Start in codelists/views/version_dmd_convert.py at the failing line, then compare the scoping approach in codelists/actions.py around lines 760-763. Update the lookup so duplicate slugs are scoped to the relevant user or organisation. Done means conversion no longer returns a 500 error and displays a banner linking to the existing DM+D codelist.