MB-5308: allow detailsCell to be computed in user-specified code
MB-5308: allow detailsCell to be computed in user-specified code

We need this in order to render better bucket details (i.e. with
compaction progress).

Change-Id: Ieada147338b6a20a9404a7fa49913f69b06fb404
Reviewed-on: http://review.couchbase.org/21222
Tested-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>
Reviewed-by: Pavel Blagodov <596b71e626fc58dbdad141b817770e3e4810bb9b@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>

mschoch/ns_server,membase/ns_server,pavel-blagodov/ns_server,pavel-blagodov/ns_server,mschoch/ns_server,daverigby/ns_server,nimishzynga/ns_server,mschoch/ns_server,pavel-blagodov/ns_server,mschoch/ns_server,t3rm1n4l/ns_server,nimishzynga/ns_server,t3rm1n4l/ns_server,ceejatec/ns_server,membase/ns_server,membase/ns_server,ceejatec/ns_server,t3rm1n4l/ns_server,membase/ns_server,vzasade/ns_server,membase/ns_server,mschoch/ns_server,vzasade/ns_server,vzasade/ns_server,ceejatec/ns_server,t3rm1n4l/ns_server,vzasade/ns_server,nimishzynga/ns_server,daverigby/ns_server,membase/ns_server,daverigby/ns_server,daverigby/ns_server,t3rm1n4l/ns_server,ceejatec/ns_server,ceejatec/ns_server,nimishzynga/ns_server,nimishzynga/ns_server,ceejatec/ns_server,daverigby/ns_server,mschoch/ns_server,nimishzynga/ns_server,pavel-blagodov/ns_server