Use completing-read instead of ido-completing-read

[pidu]

I noticed some of my own homegrown packages still using ido after switching to counsel / ivy. It's pretty nice to have the completion system be uniform. In my case I could just skip the ido- prefix and ivy-mode started working as per the docs (https://github.com/abo-abo/swiper):

ivy-mode ensures that any Emacs command using completing-read-function uses ivy for completion.

Perhaps paperless could go with completing-read for it's default as well, or make it customizable? Just a thought.