Encapsulate Issuable sidebar logic in a class

Looking at https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3256/diffs, this class has too much knowledge of the sidebar. We should create a class that encapsulates this logic.

All MergeRequestTabs should be doing is calling SomeSidebarClass.expand() and SomeSidebarClass.collapse(). It should also be the single source of knowledge about persisting the change to a cookie, and so on.

cc @jschatz1