Display component title popups properly

As noticed in code review, the refactored version of the maintenance
UI did not display the same descriptive title tooltips on component
trails as the original did.

This solves the issue by adding a separate `component_description`
template tag.

python/nav/django/templatetags/maintenance.py

@@ -46,6 +46,18 @@ def component_name(component):
     return component.pk
 
 
+@register.filter
+def component_description(component):
+    """Returns a description of a component useful as a link title tooltip.  Returns
+    an empty string if there is no further description than the component's name.
+    """
+    if isinstance(component, MissingComponent):
+        return str(component)
+    if hasattr(component, "ip"):
+        return str(component.ip)
+    return getattr(component, "description", "")
+
+
 @register.filter
 def component_db_table(component):
     """Returns the database table name of a model object used as a maintenance

python/nav/web/templates/maintenance/frag-component-trail.html

@@ -1,10 +1,10 @@
 {% load maintenance %}
 {% for element in trail %}
-    {% with element|component_name as title %}
+    {% with element|component_name as name %}
         {% if element.get_absolute_url %}
-            <a href="{{ element.get_absolute_url }}" title="{{ title }}">{{ title }}</a>
+            <a href="{{ element.get_absolute_url }}" title="{{ element|component_description }}">{{ name }}</a>
         {% else %}
-            {{ title }}
+            {{ name }}
         {% endif %}
     {% endwith %}
     {% if not forloop.last %} → {% endif %}