Include code frequency indicators in builder tool

When authoring a new codelist, there are diminishing returns on the time the author spends on getting every last code correct. On a large codelist, this long tail of apparently-very-unusual-but-possibly-significant codes can take a significant amount of time to QA, possibly with very little benefit: but no-one ever finds out if the work was useful.

It would be good to have an indicator of the frequency of each code for authors to access when making decisions about code inclusion (or exclusion).

From an IG point of view it's been suggested we just start with frequencies for codes in our studies, but this doesn't help with the primary use-case, which is helping authors of new codelists (or editors of existing ones) to make decisions about adding unusual codes.

From a research point of view there's a concern that code counts could be misused for very crude studies on their own.

For the purposes of codelist authoring, centiles of a log rank would probably be sufficient: we just want to know "this code is vanishingly infrequently used". This could probably be across a single year of data aggregated across TPP and EMIS. This may address possible concerns around data stewardship etc.

The issue names the builder tool and codelist authoring but does not identify files, tests, or an entry point. First map how codes are loaded and displayed, then resolve the data-source, aggregation, privacy, and presentation requirements; done means an agreed indicator is available to authors and covered by tests.