Modularize code to set up a proper rewrite

dangerzone-webdav-processor was renamed to dangerzone_webdav_processor/__main__.py, the remaining files were abstracted out of it add script to run the dangerzone_webdav_processor module create gitignore to ignore pip/mypy files

This is just the start of the abstraction. I wanted to modularize things to make that easier first, and this should be merged back into the main branch so there aren't more changes to the old single-file script. I've tested (both the module and wrapper) and everything works fine On My Machine™