Pre-calculate Emoji digests

By pre-calculating the digests we can manually construct the emoji URLs,
removing the need for using Rails' asset URL helpers. The reason we
don't want to use these helpers for Emojis is two-fold:

1. Rails' image_url() method is slow, really slow. For one it _might_
   have to calculate digests but it also performs a lot of other
   intensive operations (judging by the source code and based on
   measuring timings).

2. We have a lot of Emoji which coupled with the above can result in it
   taking minutes to load Emoji autocomplete data.

Using this pre-calculation setup generating the digests takes around 7
seconds (including the time it takes to start Rails/Rake), and only
around 600 milliseconds to load _all_ the autocomplete data of a project
(measured locally).

This commit _does_ change the Emoji URLs from absolute to relative URLs
as these are much easier to generate.

To update the Emoji data simply run:

    rake gemojione:digests

Then commit any changes.

Fixes gitlab-org/gitlab-ce#14009