Add a "Execute script" - interface

[torwag]

Many of the feature request deal with changes in the documents itself.
Split, Merge, Rotate, Translate,
#1294, #789, #734, #1410, #1280 to name a few.
I would like to propose that instead of adding all those features into paperless-ng natively, there could be a consumable script folder which get listed in a drop-down menu in paperless-ng. Together with a defined set of input parameters and defined outputs, people could start to contribute all kind of scripts to their particular needs. Obviously there are some security concerns, therefore, I would love to discuss the idea here.

To get this to work, we need to define how such scripts would be called,

1. The set of parameters which are given to the script (e.g. filename and location)
2. The expected return value
3. How to deal with the original documents after being processed?

If this is defined, one could select one or multiple documents, select a script (e.g. 'merge') and the script does the magic. A paperless-helper module could be defined to make e.g. scripting in python easier, as it would allow easy access to different paperless aspects (similar like the API). This helper module could also be used to enable the design of small GUIs in the script to make some parameters easier to deal with. E.g. A splitting script could have some pop-up window asking for the page at which the break should happen.

Scripts could deal with merging, splitting, watermarking, rotating, image enhancing, clipping, converting, etc.

Would that be something, people are interested in?