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

This relaxes the commit rules regarding the number of changed lines.
Previously we required a detailed description if 20 or more lines were
changed. Starting with this commit we change the rules so Danger only
notifies you about writing a detailed description if:

1. A commit changes more than 3 files
2. The same commit changes more than 30 lines

This should make it easier to do a find replace to fix a typo, without
having to repeat what the subject line already says, while still
reminding developers to write a detailed description for new features
and other larger changes.

Fixes https://gitlab.com/gitlab-org/release/framework/issues/13

This Dangerfile ensures that commit authors write good commit messages.
We also prevent the use of Emoji, since these add no value and may not
always be displayed properly.

Merge commits are currently ignored. In the future we most likely want
to stop using these, but this is a separate discussion, hence we ignore
them for the time being.

The rules enforced by this Dangerfile are based on the article found at
https://chris.beams.io/posts/git-commit/. For more information, refer to
https://gitlab.com/gitlab-org/gitlab-ce/issues/50003.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/50003