skip forbidden pages and distinguish them from empty selections

Collected pages the user may not read are dropped from the export and remembered
separately, so a selection where every page is forbid

skip forbidden pages and distinguish them from empty selections

Collected pages the user may not read are dropped from the export and remembered
separately, so a selection where every page is forbidden can be told apart from
one where nothing was selected and reports "no read access" instead of "nothing
selected". The forbidden message no longer lists the page ids or references the
removed 'Skip Forbidden Pages' option; its outdated translations are dropped so
they get re-translated.

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give user feedback for empty or invalid PDF exports

Collectors used to return an empty page list for a missing page, a missing
namespace or an empty selection, which then failed silently or crashed

give user feedback for empty or invalid PDF exports

Collectors used to return an empty page list for a missing page, a missing
namespace or an empty selection, which then failed silently or crashed when the
first page was accessed. They now throw a translated ExportException instead,
which the action handler turns into feedback: a flash message and redirect for
regular navigations, or an HTTP error with the message in the body for the
background download BookCreator uses. Dynamic message parts are HTML escaped
because that body is injected into the page via a jQuery html() sink.

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Use safer array access

Set up book title to be available for placeholder replacement

Rename DokuPdf class to avoid accidental deletion based on deleted.files

deleted.files includes DokuPDF already, and on some systems the extension manager might delete DokuPdf as well

Use caching settings

make PdfExportService reusable in debug mode

This refactors the service so that tests can easily access the produced
debug HTML

extracted PDF generation and sending out of action

Now only event handling happens in the action handler