emoji: Add support for animated GIF images.

[HarshitOnGitHub]

This commit adds 'resize_gif()' function which extracts each frame,
resize it and coalesces them again to form the resized GIF while
preserving the duration of the GIF. I read some stackoverflow
answers all of which were referring to BiggleZX's script
(https://gist.github.com/BigglesZX/4016539) for working with animated
GIF. I modified the script to fit to our usecase and did some manual
testing but the function was failing for some specific GIFs and was not
preserving the duration of animation. So I went ahead and read about
GIF format itself as well as PIL's GifImagePlugin code and came up
with this simple function which gets the worked done in a much cleaner
way. It tested this function on a number of GIF images from giphy.com
and it resized all of them correctly.

Fixes: #9945.

[zulipbot]

Hello @zulip/server-emoji members, this pull request was labeled with the "area: emoji" label, so you may want to check it out!

[HarshitOnGitHub]

@timabbott FYI. Mypy is complaining because there are no stub files available for GifImagePlugin.

[HarshitOnGitHub]

@timabbott Gentle ping on this. What can I do to fix the mypy issue?

[timabbott]

For the mypy error, there are two options. Either edit mypy.ini to add an ignore_missing_imports block, or add stubs like what we have in the stubs/ directory, for the PIL function we're using.

[timabbott]

Nice, merged, thanks @HarshitOnGitHub!

[julievalet]

Nice job on the resizing !
One question: it was working fine for a while and now, every time I upload a new custom GIF emoji, the background, which should be transparent, is now black.
Do you think it would come from the resizing which doesn't keep the alpha ?

[timabbott]

That seems quite likely. @julievalet can you open a new issue for that and mention @HarshitOnGitHub on the thread?

[julievalet]

Here is the created issue : #10351