Add "working tag" option

[lippoliv]

Beside the "query mode" (e.g. newest or updated) we should introduce a "working tag" option.

The script would look up all documents with this tag and transfer them to sevDesk. After downloading from paperless, the tag would be removed by the script, so it wouldn't be transferred twice.

This way users can, at any time, flag documents that need to be transferred to sevDesk, whenever they work in Paperless.