Improve Emoji detection in commit messages

This changes our Danger integration so that it is smarter about
detecting GitLab emoji in commit messages. This is done using a two step
process:

1. We use the old regular expression to determine if a commit message
   _might_ include an emoji.

2. If this regular expression matches, we then check for all existing
   emoji names in the commit message. This might be a wee bit expensive,
   but because we couple it with the previous step it should be rarely
   executed.

This ensures we don't trigger an error when a commit message includes
text such as `:foo:bar:`, which is not a valid Emoji.

Fixes https://gitlab.com/gitlab-org/release/framework/issues/15

danger/commit_messages/Dangerfile

@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+require 'json'
+
 # rubocop: disable Style/SignalException
 # rubocop: disable Metrics/CyclomaticComplexity
 # rubocop: disable Metrics/PerceivedComplexity
@@ -8,6 +10,30 @@
 # https://github.com/jonallured/danger-commit_lint because its output is not
 # very helpful, and it doesn't offer the means of ignoring merge commits.
 
+class EmojiChecker
+  DIGESTS = File.expand_path('../../fixtures/emojis/digests.json', __dir__)
+  ALIASES = File.expand_path('../../fixtures/emojis/aliases.json', __dir__)
+
+  # A regex that indicates a piece of text _might_ include an Emoji. The regex
+  # alone is not enough, as we'd match `:foo:bar:baz`. Instead, we use this
+  # regex to save us from having to check for all possible emoji names when we
+  # know one definitely is not included.
+  LIKELY_EMOJI = /:[\+a-z0-9_\-]+:/
+
+  def initialize
+    names = JSON.parse(File.read(DIGESTS)).keys +
+      JSON.parse(File.read(ALIASES)).keys
+
+    @emoji = names.map { |name| ":#{name}:" }
+  end
+
+  def includes_emoji?(text)
+    return false unless text.match?(LIKELY_EMOJI)
+
+    @emoji.any? { |emoji| text.include?(emoji) }
+  end
+end
+
 def fail_commit(commit, message)
   fail("#{commit.sha}: #{message}")
 end
@@ -33,6 +59,7 @@ end
 
 def lint_commits(commits)
   failures = false
+  emoji_checker = EmojiChecker.new
 
   unicode_emoji_regex = %r((
     [\u{1F300}-\u{1F5FF}] |
@@ -117,7 +144,7 @@ def lint_commits(commits)
       failures = true
     end
 
-    if commit.message.match?(/:[\+a-z0-9_\-]+:/)
+    if emoji_checker.includes_emoji?(commit.message)
       fail_commit(
         commit,
         'Avoid the use of Markdown Emoji such as `:+1:`. ' \