Make shortcut list available from UI

Twitter: https://twitter.com/connorjshea/status/714703319713886209

We currently have some documentation on this under http://doc.gitlab.com/ce/workflow/shortcuts.html but this is not easily visible

The shortcut list is triggered by pressing 'Shift + ?'. Very many users are however not aware that this feature exists.

Can we integrate the shortcut list to the UI under Shortcuts link that a user can click to get a list of shortcuts?

cc/ @sytses @JobV @connorshea