Change icon for right sidebar toggle

Description

The icon in the right sidebar toggle differs from that of the toggle in the left sidebar.

Proposal

Use the same icon as the sidebar collapse/expand toggle in the left sidebar, but in the upper-right toggle for the right sidebar.

This should be applied to all collapsible right sidebars for Issues, Merge Requests, Milestones, and Epics.

Links / references

Proposed by @dimitrieh via https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18135#note_66758197