Add 'Uploads' hub and navigation

The uploads hub is just a page with text for now - there are no actions
available on the page. It is linked to from a new 'Uploads' menu item on
the left of the page which is only visible if your service has the
`letter` and `upload_letters` permissions and if the current user has
permissions to send messages.

app/main/__init__.py

@@ -33,6 +33,7 @@ from app.main.views import (  # noqa isort:skip
     styleguide,
     templates,
     two_factor,
+    uploads,
     user_profile,
     verify,
 )