danger: Allow section prefix in commit messages

What does this MR do?

This allows to start commit messages with a "section" prefix (e.g. danger: My commit message), while still keeping the capitalisation requirement for the message part (e.g. My commit message here).

Prefixing commit messages with prefixes is a common practices among big projects (e.g. Git):

Git, but also other projects, require commit messages to start with the
section in which the changes was made. This is helpful when running for
example `git log` on a directory, as by just reading the first word of
the commit subject, a lot of commits do not require reading the rest of
the message.

This is a smaller change compared to !29136 (closed), but still allows the same use-case.