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
15 jobs for precompile-emoji-urls
Status Job ID Name Coverage
  Test
passed #963604
ruby mysql
brakeman

00:14:09

passed #963607
ruby mysql
flay

00:12:41

passed #963605
ruby mysql
flog

00:11:31

passed #963601
ruby mysql
rubocop

00:02:42

passed #963602
ruby
scss-lint

00:02:27

passed #963589
ruby mysql
spec:api

00:26:13

passed #963588
ruby mysql
spec:feature

00:43:55

passed #963591
ruby mysql
spec:lib

00:25:56

passed #963590
ruby mysql
spec:models

00:18:38

passed #963593
ruby mysql
spec:other

00:32:54

passed #963592
ruby mysql
spec:services

00:47:05

passed #963597
ruby mysql
spinach:other

00:17:40

passed #963594
ruby mysql
spinach:project:half

00:41:01

passed #963596
ruby mysql
spinach:project:rest

00:22:29

passed #963598
ruby mysql
teaspoon

00:02:56