5dab6e9d31 began honoring the list of disposals for every frame.
Unfortunately, passing a list of disposals for a non-animated image
raises an exception:
```
File "zerver/lib/upload.py", line 212, in resize_emoji
image_data = resize_gif(im, size)
File "zerver/lib/upload.py", line 165, in resize_gif
frames[0].save(
File "[...]/PIL/Image.py", line 2212, in save
save_handler(self, fp, filename)
File "[...]/PIL/GifImagePlugin.py", line 605, in _save
_write_single_frame(im, fp, palette)
File "[...]/PIL/GifImagePlugin.py", line 506, in _write_single_frame
_write_local_header(fp, im, (0, 0), flags)
File "[...]/PIL/GifImagePlugin.py", line 647, in _write_local_header
disposal = int(im.encoderinfo.get("disposal", 0))
TypeError: int() argument must be a string, a bytes-like object or a
number, not 'list'
```
`check_add_realm_emoji` calls this as:
```
try:
is_animated = upload_emoji_image(image_file, emoji_file_name, a
uthor)
emoji_uploaded_successfully = True
finally:
if not emoji_uploaded_successfully:
realm_emoji.delete()
return None
# ...
```
This is equivalent to dropping _all_ exceptions silently. As such,
Zulip has silently rejected all non-animated images larger than 64x64
since 5dab6e9d31.
Adjust to only pass a single disposal if there are no additional
frames. Add a test for non-animated images, which requires also
fixing the incidental bug that all GIF images were being recorded as
animated, regardless of if they had more than 1 frame or not.
Alex Vandiver