Warn when MR titles are too long

This adds a Danger check to produce a warning for MR titles greater than
72 characters. This is necessary because when squash on merge is used,
the MR title will be used for the commit message (at least by default).
This in turn leads to commit messages that violate our commit message
guidelines.

For more information, refer to
https://gitlab.com/gitlab-org/release/framework/issues/24.

danger/metadata/Dangerfile

@@ -23,3 +23,10 @@ has_pick_into_stable_label = gitlab.mr_labels.find { |label| label.start_with?('
 if gitlab.branch_for_base != "master" && !has_pick_into_stable_label
   warn "Most of the time, merge requests should target `master`. Otherwise, please set the relevant `Pick into X.Y` label."
 end
+
+if gitlab.mr_json['title'].length > 72
+  warn 'The title of this merge request is longer than 72 characters and ' \
+    'would violate our commit message rules when using the Squash on Merge ' \
+    'feature. Please consider adjusting the title, or rebase the ' \
+    "commits manually and don't use Squash on Merge."
+end