Make sorting dropdows consistent

Description

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14320 introduced some improvements to the sorting dropdown menu for issuables (issues list page and merge requests page).

Before	After

Since the MR is merged, now our sorting dropdowns are a bit inconsistent.

Proposal

Let's add the selected mark to remaining sorting dropdowns.
We should use the sortable_item helper for sortable items in menus, and add the .dropdown-menu-selectable to the items wrapper in haml files.

@sarrahvesselov @tauriedavis what do you think about it? 🙂