Print multiple documents at once

[horcsinbalint]

We got a feature-request that it would be a nice to allow multiple PDF files to be uploaded and printed with the same settings by only submitting the printing form once.

It is probably possible by changing resources/views/dormitory/print/print.blade.php to allow multiple files to be uploaded and to go through the array of files one-by-one in app/Http/Controllers/Dormitory/PrintController.php.
But I do not see an exact way of how we should send feedback to a user.

I had an idea of merging the PDFs on client-side with something like this: https://pdf-lib.js.org/ But it would come with its own set of issues, like having to concatenate PDFs with an odd number of pages when dual-page printing is required, and how the price calculations should work in these cases.

Also we could probably do the merging on server-side and merge the PDFs together and then send off to the printer. Would still only be a single print job, so giving feedback would be manageable. We would still need to add empty pages but at least the price could be calculated without issues.

[kdmnk]

Tbh I would not prioritise this before #159. We might have some serious refactoring to do, and we can involve this one there.