Assign codes to multiple codelists

Currently on OpenCodelists we can create a a single codelist and then search for a single code at a time. However there are two situations that occur frequently

• We have a single code that we know will be assigned to multiple codelists
• We are need multiple codelists for a similarish topic which have a lot of overlap

This related to #507 where we had proposed categories.

Ethnicity
In the early days of OpenSAFELY this occured with
https://codelists.opensafely.org/codelist/opensafely/ethnicity/2020-04-27/#full-list where we applied a ctagory. It might be nice in the UI to assign a code to multiple codelists at the same time if a category not appropriate. The same for diabetes medication mentioned in #507

Codelists for similarish topics with overlap
We want to identify "probable covid" - we know that there will be three distinct ways of doing this; test, clinical code, covid sequalae and we also know they will overlap. We have generated a list of all possible codes and are assigning them in a google sheet. This works fine although a bit slower than if you could assign in the UI. The major risk of working outside the UI here is due to the length of SNOMED codes and programmes like excle truncating them

Start by reviewing issue #507 and the ethnicity and probable-COVID examples described here, including the linked codelist and spreadsheet. Define the UI workflow for assigning one code to multiple codelists and handling overlapping codelists; done means the assignments can be made in OpenCodelists without the spreadsheet workaround and full SNOMED codes remain intact.