MB-8662: implemented ui for purge interval setting

Change-Id: I6f114cf67e86613b236d89f4bf943e4c1e6d6036
Reviewed-on: http://review.couchbase.org/27794
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>