Update sidebar Vue components to use single mediator/store

There are currently two different patterns for components in the issuable sidebar. We should go with the single mediator/store pattern which gets passed in.

1. Creates their own mediator/store
   • sidebar_assignees
   • sidebar_time_tracking
2. Passes in a single mediator/store
   • lock_issue_sidebar.vue
   • confidential_issue_sidebar.vue

---

From some discussion here, https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14836#note_44473754