refactor: remove some master backlog calculation loops and do not request closed sprint data if not enabled

handle nil argument for backlogs_by_prints/releases
make rank use scoped list_with_gaps_scope_condition
use scope chain class method to inject higher/lower items
compact last_updated array before sort
TEST: simple benchmark on mbp-controller
do not render higher_item_id on backlog std load
load tooltips via ajax, speeding up backlog loading. #698
Load closed sprints on request using ajax
render model_errors only if there are errors
remove tooltip from sprint title, it is not useful. TODO: show complete sprint information
user feedback if no closed sprints to show
Revert "TEST: simple benchmark on mbp-controller"

This reverts commit df4549aaeb83edffa8a0100b4ca7ef8749ec69bb.
we need to drop support for javascript engines not supporting "trim" on strings
fix tests for ajax loading completed sprints
Config option for scrum statistics menu entry, fixes #839
Merge branch 'optimize/loadingtime'
Merge branch 'feature/839-scrum_stats_menu'