feat: Added handling of non-downloadable filter lists to user interface [#1328]

This MR contains the following changes:

• In desktop settings page:
  • Show "Bundled" text as last updated version for non-downloadable filter lists.
    Note that it's not clear yet how Subscription objects will look like after EWE starts supporting dynamic filter list updates for bundled filter lists. Until then, filter lists are either bundled or updated dynamically, so there's no way to determine yet whether a bundled filter list wasn't updated yet. Therefore we'll need to revisit this logic in #1361 (closed).
  • Hide "update" button from filter list context menu for non-downloadable filter lists.
• Made <subscription downloadStatus> property in issue report data optional.
  In the web interface it is shown as "unknown" (see test issue report).