Enh: domain list sorting should take account of level

The domain list sorting is currently alphabetical by full domain.

The fact that the main 'apache.org' entry occurs in the first tab is purely because apache starts with 'a'. Since the top-level domain is likely to be special in any mail archive this should be allowed for.

There are various ways to do this.
- split the domain path before sorting, and treat a missing segment as being early in the alphabet
- list the TLD(s) in a separate tab (default to previous method if using the 'table' style)

Inspect the domain-list sorting entry point and how the first tab and table style are selected. Compare the proposed segment-aware ordering with the separate-TLD-tab option, then verify that the main domain and subdomains appear in the intended tabs and order.

Written by the indexing model from the issue text.