Begin replacement of server-rendered context menu

Removes WorksPackages::ContextMenusController, its views and specs, and
ContextMenuHelpers.

This patch begins the work needed to serialise a given WorkPackage's
'actions' in the v3 API response, based on a user's permissions. This
should support a client-rendered context menu (or any other contextual
UI for that matter).

Much code was simply copy-and-pasted from ContextMenusController into
Api::V3::WorkPackagesController. This code still requires considerable
refactoring and optimisation.

Signed-off-by: Alex Coles <alex@alexbcoles.com>