Enh: when using phonebook style front page, drop letters that have no entries

Note: this would not apply to lists.apache.org which currently has lists for all letters.

[Potential implementation note: the tab widths are 24px and the gaps are 2px; phonebook_page.style.width needs to be set accordingly.]

Locate the phonebook-style front-page implementation and inspect how its tabs and entries are assembled. Preserve the exception for lists.apache.org, account for the 24px tabs and 2px gaps when sizing phonebook_page.style.width, and verify that letters without entries are omitted elsewhere.

Written by the indexing model from the issue text.