Switch from macros to blocks for navigation menus

This affects the user and notifications menus. It happens for two reasons:
- the only argument passed to the macros was current_user, which is always available
- we want to enable overriding and adding items to the menus via extensions

At the moment only the user menu takes advantage of the base template, since the blender-cloud extension makes use of it, while notifications.pug does not need it yet.

src/templates/menus/user.pug

@@ -0,0 +1 @@
+| {% extends 'menus/user_base.html' %}