tootctl emoji import: case insensitive duplicate check

[angristan]

I added a few hundreds of emojis to one of my instances using tootctl emoji import and tar.gz files.

Some of them had duplicates with different case, like this:

They are displayed correctly in the admin and don't cause any issue during import.

However they don't display correctly:

So I ended up removing the duplicates and reuploading the ones that had the same shortcode with a different case but a different image.

As explained by someone on discord (not sure if they want their name to be quoted)

the db schema for custom emoji is case sensitive, but CustomEmoji .search uses SQL's ILIKE operator it's not case sensitive.
this means you can add emojis as case sensitive, but it can't be used as case sensitive.

To a least fix the batch import, I have come up with the fix attached to this PR which check of existing emojis regardless of the case.

That fixed the issue for me, for example if I have the LoL emoji, importing a tarball containing lol.png worked, but with the PR it would skip it.