Link to upload data page in navigation bar

Everyone who can sign in can see this, so there's no need to wrap
it in logic.
Add @requires_permission decorator

This decorator takes a string and either redirects the user or
allows them to continue on with their request.
Add view helper to conditionally show navigation

We want to show a link to allow the user to edit dashboards, but
only if they have that permission.
Add dashboard index page

This page will list dashboards that the user is able to edit.

The page itself and the menu navigation for it are only visible to
users who have the 'dashboard-editor' permission.
Merge pull request #46 from alphagov/73958164-flag-dashboard-admin

Add a flag to conditionally display dashboard administration