webadmin: refactor navigation bar

This patch makes several minor improvements to the navigation bar.

  1. The unavailable page is removed; instead, disabled items (e.g.,
     pool queue plots and billing plots, with default configuration)
     appear as (unclickable) text rather than a link to the non-descript
     unavailable page.

  2. The text for disabled items (see above) has specific CSS class;
     this allows the CSS to show the text as ghosted out items, giving
     a visual hint that these items are not available.

  3. The HTML 'title' attribute, which browsers typically show as a
     mouse-over tool-tip, provides specific information on why the
     item is disabled.  This gives the admin a hint how to enable
     such disabled items.

  4. The pages that require authentication (pool admin, cell admin,
     and alarms) have an additional CSS class when user is currently
     not authenticated.  With this patch CSS makes such text a dark
     red.  As before, clicking on these link redirects the user to the
     login page.  Once logged in, these links become the normal
     (azure/blue) colour and clicking on the link takes the user to
     the expected page.

  5. The a-tags in the nav-bar now have id elements.  This is to aid
     functional testing.

  6. The text for the link names (stored in BasicNavigationPanel.properties)
     are now reference the link's id, rather than by index.

The patch also refactors the corresponding BasicNavigationPanel class.

Target: master
Requires-notes: no
Requires-book: no