UI: Issuable list item tweaks

Depends on !1953

See the commits for more details, the messages mostly speak for themselves.

Before:

![Screen_Shot_2015-12-02_at_17.37.40](/uploads/00ac22d131c49ce548bccd96866cbc75/Screen_Shot_2015-12-02_at_17.37.40.png)

After:

![merge_item_after](/uploads/201d742494596c50bbdb3626f06f65a8/merge_item_after.png)

Fixes https://dev.gitlab.org/gitlab/gitlabhq/issues/2590 and https://dev.gitlab.org/gitlab/gitlabhq/issues/2589

See merge request !1962