Update custom emoji to work on all backgrounds

[SomniusTiger]

All-black emoji do not show up on black backgrounds, and all-white emoji do not show up on white backgrounds.

We should include borders or drop shadows to make these visible across all types of backgrounds, not just medium greys!

[drisc]

I'll have a go at it, could you commit the images to the repo?

[SomniusTiger]

@drisc Oh, so, custom emoji are uploaded through the admin UI, which @neauoire and I have access to! I can upload the images to update them, but first I need the updated images to do so :3

[drisc]

I still need the original images to begin the process, I can download them all via 'Inspect Element' but if they are in the repo we can track all the updates here.