[WIP] Only resize large GIFs when neccessary to avoid breaking alpha

[moagstar]

Added some logic to the custom emoji resizing code to prevent resizing in certain circumstances. This is because the pillow gif handling code seems to be rather flaky with regards to handling gif color palletes, causing broken gifs after resizing. The workaround is to only resize when absolutely necessary, which is basically whenever not resizing wouldn't cause bandwidth issues (size, file size). Non square images are always resiezd.

#10351

Testing Plan:
I added some tests to EmojiTest and tested manually.

I discussed this approach on chat.zulip.org and there seems to be consensus that this is a worthwhile approach. Now I am looking for feedback on the actual values chosen.

[timabbott]

From looking https://slackmojis.com/ (a random website I found with lots of custom emoji), most custom emoji files folks might try to upload are probably bigger than 16KB. Based on that analysis, I think doing something like 128KB as the limit here would be reasonable. 128 seems right for max size.

[timabbott]

@moagstar this looks reasonable, thanks for doing this! Can you clean up the linter errors and adjust the constants as suggested above? After those changes, I'm happy to merge this.

[timabbott]

Oh, also, you should clean up the commit message to line-wrap properly and otherwise follow our conventions (compare in git log and you'll see what I mean). Check out the Zulip commit message guidelines for more details: https://zulip.readthedocs.io/en/latest/contributing/version-control.html#commit-messages

[moagstar]

@timabbott Thanks for your review and link re commit messages. I have fixed the linter errors and failing test now.

[timabbott]

Nice, merged as 1e65cdd, after rewriting the commit message to be more readable to folks who didn't work on this issue. Thanks for fixing this @moagstar!! I think a lot of users will appreciate this :)

[sjuxax]

@moagstar Is there a particular reason that you're resizing any non-square gif? Consider https://slackmojis.com/categories/7-party-parrot-emojis, in which many of the gifs are non-square, but otherwise meet the constraints fine.

[moagstar]

@sjuxax this was one of the requirements as discussed on github and chat.zulip.org