repo didn't have it already

Tried to modify global STPKG_TOKEN in subshell in __upload_file, so
didn't work

Fix gif cropping for weird gifs
Remove gif optimization as discord doesn't know how to display them
Fix output dimensions for pictures with big alpha borders

Still needs debugging and all

No functional changes

Also allow change of stickerpicker repository.

This fixes `stpkg pack` accepting token with `-t` but not `stpkg edit`

Show help and exit if no config file is found

Fix configuration generation

Conversion is still done to png if given file is not png of gif

Just in case: after an edit, the json in the exposed picker is not
updated, you may need to stpkg del && stpkg add it to take into account
the modifications.

(to prevent concatenation of sizes)