Allow downloads of ICD-10 codelists with HES-specific "X" padding

Why are we doing this?

Three-character ICD-10 codes are appended with an "X" to four character codes in HES/SUS.
This affects the APCS dataset in OpenSAFELY-TPP.
There is an existing issue for how to handle this in ehrQL but it is non-trivial.

We could handle this in OpenCodelists in a manner akin to how we do for the VMP mapping issue in dm+d codelists.

How will we know when it's done?

The default download for an ICD-10 codelists as used by the opensafely CLI tool contains three-character codes (if present in the codelist) both in their original form and with an "X" appended.

There are available two download buttons on an ICD-10 CodelistVersion page - one for the codelists without the appended "X"s and one with.

What are we doing?

1. Modify codelists/models.py::CodelistVersion.csv_data_for_download to have an optional kwarg of omit-hes-appended-x, add logic to conditionally append "X" to three-character codes in the returned csv data if this kwarg is True and the coding system is icd10.
2. Modify codelists/views/version_download.py::version_download to handle a URL parameter of omit-hes-appended-x.
3. Modify templates/codelists/version.html, specifically the div with a class of version_sidebar, to add a conditional block which checks if clv.coding_system == "icd10" which writes two buttons of class btn btn-outline-primary.
   1. The first will have an inner text of Download CSV <small class="d-block"> with HES/SUS X appended*<small> and an href of "{{ clv.get_download_url }}".
   2. The second will have an inner text of "Download CSV <small class="d-block"> original</small>" and an href of "{{ clv.get_download_url }}?omit-mapped-vmps"
4. Add an explanation of the "X" padding behaviour to the APCS page on OpenSAFELY docs
5. Modify templates/codelists/_about_tab.html to include the following in the div of class small pt-4 :

{% if codelist.coding_system_id == "icd10" %}
      <p>
        * ICD-10 codelists can be downloaded with three-character codes padded to four-characters with an "X" at the end - this is the default
        for an OpenSAFELY project.  See the <a href="https://docs.opensafely.org/data-sources/apc/">
          documentation the APCS data source</a> for more details.
      </p>
{% endif %}

Defining delivery tasks guidance

Start with codelists/models.py::CodelistVersion.csv_data_for_download and codelists/views/version_download.py::version_download, then inspect the version_sidebar in templates/codelists/version.html and templates/codelists/_about_tab.html. Check the APCS documentation page requirements and existing download behavior. Done means ICD-10 downloads offer both padded and original forms, the default includes HES/SUS X padding, and the related explanations appear in the UI and APCS documentation.