allow custom oneko image via themes - fixes #472

What does this PR do?

after this change, one can set a custom image by:

• upload an appropriate image to the drive (some images can be found at https://github.com/vencordcss/onekocord/tree/main/onekoskins), possibly with the "keep original" option set
• copy the URL to the image in the drive
• create/edit a theme so that it contains (inside props):

  "oneko-image": '"url(https://yourinstance.example.com/files/ee17b385-a084-4e2a-b531-225dfb96cc3c)',

  with the proper URL (yes, the double quote at the start is necessary to tell the parser it's not a colour)

That's it!

Of course I'll update the theming documentation once this is merged.

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