Setting a channel thumbnail as an SVG via ricecooker breaks publishing

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This seemed to have been caused by using an SVG image as the thumbnail for a channel. It's possible this would impact other thumbnails too. Ideally, setting as an SVG would be fine, and it would just be appropriately converted to a PNG on publish.

Sentry Issue: STUDIO-HV3

UnidentifiedImageError: cannot identify image file <File: /tmp/tmp8glzgicl>
(3 additional frame(s) were not displayed)
...
  File "contentcuration/viewsets/channel.py", line 528, in publish
    channel = publish_channel(
  File "contentcuration/utils/publish.py", line 935, in publish_channel
    set_channel_icon_encoding(channel)
  File "contentcuration/utils/publish.py", line 759, in set_channel_icon_encoding
    channel.icon_encoding = convert_channel_thumbnail(channel)
  File "contentcuration/utils/publish.py", line 779, in convert_channel_thumbnail
    return get_thumbnail_encoding(channel.thumbnail)
  File "contentcuration/utils/files.py", line 112, in get_thumbnail_encoding
    with Image.open(inbuffer) as image:

Start with contentcuration/utils/files.py at get_thumbnail_encoding, then follow the publish path through contentcuration/utils/publish.py and contentcuration/viewsets/channel.py. Reproduce the channel-thumbnail publish failure with an SVG and verify that SVG thumbnails are converted appropriately before encoding, while checking whether other thumbnails are affected.