Webapp redesign phase 4: Updates overview and rollback review

- GET /updates/: one card per updater (Dell, iPXE, Blancco, BPRT) with
  its state, a Check now / Open action and details in accordions; the
  shared download cap is a small popover.
- Dell applies list: one line per apply titled by its first change
  ('Latitude 5350 pack A11 to A12', model looked up in the maps) and
  'and n more'; the chip says how many written files are unchanged since
  the apply. The body lists every change, skipped item, error and file.
- dell_updates.file_status(): per file an apply wrote, unchanged /
  changed / missing by content hash, the same test rollback uses.
  describe(), history_view() and rollback_preview() build on it.
- GET /updates/dell/rollback/<id>: review a rollback before doing it,
  with the files that would be restored and a diff of each. The POST
  route is unchanged; both forms can return to the overview.

webapp/templates/base.html

@@ -66,7 +66,8 @@
       </div>
       <div class="ui-group">
         <div class="ui-navhead">Updates</div>
-        {{ navlink('Dell drivers and BIOS', url_for('dell_updates.page'), ep.startswith('dell_updates.')) }}
+        {{ navlink('Updates overview', url_for('updates_overview'), ep in ('updates_overview', 'dell_updates.rollback_review')) }}
+        {{ navlink('Dell drivers and BIOS', url_for('dell_updates.page'), ep.startswith('dell_updates.') and ep != 'dell_updates.rollback_review') }}
         {{ navlink('iPXE boot files', url_for('ipxe.page'), ep.startswith('ipxe.')) }}
         {{ navlink('Blancco releases', url_for('blancco_update.page'), ep.startswith('blancco_update.')) }}
         {# Plain href: the page still renders if views.bprt_sync is not registered yet. #}