Invalid items are those that are not ready, and not shown to the user.
When changing metadata, it should only be changed, if the item was not
already shown to the user.
The job executor depends on backend module, since it may control the
application via user tasks. The `ONode` can now be moved from the
store module into the backend module.
Each attachment is now first extracted into potentially multiple ones,
if it is recognized as an archive. This is the first step in
processing. The original archive file is also stored and the resulting
attachments are associated to their original archive.
First support is implemented for zip files.
Periodic tasks are special in that they are usually kept around and
started based on a schedule. A new component checks periodic tasks and
submits them in the queue once they are due.
In order to avoid duplicate periodic jobs, the tracker of a job is
used to store the periodic job id. Each time a periodic task is due,
it is first checked if there is a job running (or queued) for this
task.
The restriction that only pdf files can be uploaded is removed. All
files can now be uploaded. The processing may not process all. It is
still possible to restrict file uploads by types via a configuration.
First, files are be converted to PDF for archiving. It is also easier
to create a preview. This is done via the `ConvertPdf` processing
task (which is not yet implemented).
Text extraction then tries first with the original file. If that
fails, OCR is done on the (potentially) converted pdf file.
To not loose information of the original file, it is saved using the
table `attachment_source`. If the original file is already a pdf, or
the conversion did not succeed, the `attachment` and
`attachment_source` record point to the same file.
