See also:
https://daniel.haxx.se/blog/2023/01/08/copyright-without-years/
https://hynek.me/til/copyright-years/

We can't have a pack that is neither sticker nor emoji. Which is why
none defaults to both on. That wasn't propagated to the UI, which made
the interaction very confusing. It also made some states unsettable,
since you can't turn anything off from the none state.

fixes #1152

- add missing mimetype
- allow removal of images from pack
- allow GIF as a format
- don't divide size by 2 if the image is very small already