www: add optional sortorder config for listing page ordering

www: add optional sortorder config for listing page ordering

Add a publicinbox.<name>.sortorder config key to control inbox
display order on the main listing page.  Lower values appear first,
inboxes without sortorder fall back to mtime-descending order below all
pinned inboxes.  Works with both direct config listing and extindex
all" paths.  Includes documentation in public-inbox-config(5) and
tests.

In my use case, we are hosting our list, and also mirroring the
lore.kernel lists. I want to pin our list to the top, because its the
main purpose of our installation.  I am running with this patch in
"https://lore.kernel.ime.usp.br/" where "kernel.ime" (our list) is
pinned to the top.

Reviewed-by: Eric Wong <e@80x24.org>
Signed-off-by: Rafael Passos <rafael@rcpassos.me>