Create a static file for emoji autocomplete
At the moment, emoji autocomplete is part of the main autocomplete response. It's about 270 KB, which is way too much to be sending each time.
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6856 is splitting this into its own controller action, which is a definite improvement. But this is static information, which we get from a gem. So why don't we build a JSON asset containing this information, with URLs to the emoji images? That asset can go through the asset pipeline and gets its own hash, so we can cache it forever.
We only need to tell the frontend about the single JSON file location, which I'm not sure about the best way to do, but is definitely going to be faster than going through Rails each time.
I'm going to CC people who've touched this in the past, even if they don't care now