Move issues sort logic to Issuable concern

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Move issues sort dropdown to shared partial

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Add sort dropdown to merge requests list

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>