Store/show shortlisted reports offline.

This:

* On an online visit to /my/planned, caches all shortlisted reports,
  their images and static maps in localStorage, with progress banner;

* When a report is added/removed from the shortlist, caches/de-caches
  that report;

* When viewing a report page offline, shows that page from the cache
  if present (replacing the dynamic map with the cached static map, and
  replacing report images with their cached equivalents – it is a shame
  to duplicate, but we cannot rely on the browser cache having these images);

* When viewing another page offline, shows an error message but also the
  list of shortlisted reports that are cached (again, replacing their
  thumbnail images with the cached versions).

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