Allow conversion from pdb

[koca2000]

Is your feature request related to a problem? Please describe. It is possible to upload pdb files by adding the extension to the list of allowed extensions but it is not possible to convert it to e.g. EPUB although it is supported by the Calibre converter.

Describe the solution you'd like An ability to convert pdb files using the Calibre converter from the Calibre-Web GUI. Since the Calibre ebooks converter supports pdb files I thing it should be sufficient to just add the extension to the list: https://github.com/janeczku/calibre-web/blob/2303fc08148de0bb09d1a41e88da314de0b6f9de/cps/constants.py#L139

[koca2000]

I tested the suggested solution and it works flawlessly.