Warn when MR titles are too long.

What does this MR do?

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.

Example:

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/release/framework/issues/24

Does this MR meet the acceptance criteria?

• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides