js: add dynamic age update

js: add dynamic age update

This patch updates the emitted "ages" dynamically on the client side.

After updating on completion of the document load, it sets a timer
to update according to the smallest age it found.  If there are any
ages listed in minutes, then it will update again in 10s.  When the
most recent age is in hours, it updates every 5m.  If days, then
every 30m and so on.

This keeps the cost of the dynamic updates at worst once per 10s.
The updates are done entirely on the client side without contact
with the server.

To make this work reliably, since parsing datetimes is unreliable in
browser js, the unix time is added as an attribute to all age spans.

To make that reliable cross-platform, the unix time is treated as a
uint64_t when it is formatted for printing.

The rules for display conversion of the age is aligned with the
existing server-side rules in ui-shared.h.

If the client or server-side time are not synchronized by ntpd etc,
ages shown on the client will not relate to the original ages computed
at the server.  The client updates the ages immediately when the
DOM has finished loading, so in the case the times at the server and
client are not aligned, this patch changes what the user sees on the
page to reflect patch age compared to client time.

If the server and client clocks are aligned, this patch makes no
difference to what is seen on the page.

Signed-off-by: Andy Green <andy@warmcat.com>
Signed-off-by: Christian Hesse <mail@eworm.de>