Enhance: archive calendar display to show the number of mails for each month

The archive calendar display on the lhs should show the number of mails for the month.

The oldest month buttons could be dropped if there is no data (e.g. list had not started).
Or at least the buttons should show a different colour if there are no mails.

No file, test, or entry point is named. Locate the archive calendar display on the left side of the JavaScript application, then trace how month buttons obtain mail data; done means months show their mail counts and months without data are either omitted or visibly distinguished.

Written by the indexing model from the issue text.