Add a configurable default tag for the dashboard

In tags mode the dashboard always opened showing every link. This adds a
"Default tag" setting (Advanced) that pre-selects one tag group on load, so
the dashboard opens filtered to it - the built-in equivalent of the custom
JavaScript workaround people have been sharing.

The setting is a select populated from the user's own tags, following the
same dynamic-option pattern already used for the search provider. When a tag
is chosen its slug is exposed on the tag list and the matching tab is
activated on load; when the setting is empty, behaviour is unchanged and all
links are shown.

Resolves #1556

lang/en/app.php

@@ -29,6 +29,7 @@ return array (
   'settings.custom_css' => 'Custom CSS',
   'settings.custom_js' => 'Custom JavaScript',
   'settings.treat_tags_as' => 'Treat Tags As:',
+  'settings.default_tag' => 'Default tag',
   'settings.folders' => 'Folders',
   'settings.tags' => 'Tags',
   'settings.categories' => 'Categories',