Up-to-date check: reduce the 'Remind me later' period to 24 hours

It used to be one week...

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Up-to-date check: suggest to run the updater at most once per 24 hours

This addresses http://trac.imagej.net/ticket/1839.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge branch 'updater-nag'

With this branch merged, the updater will suggest to open the updater
at most once within any 24h period.

This addresses http://trac.imagej.net/ticket/1839.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>