Add check_branch_conventions CI checks

[peterbarker]

            self.check_merge_commits(),
            self.check_fixup_commits(commits),
            self.check_commit_lengths(commits),
            self.check_author_emails(),

As opposed to ArduPilot's main repo checks:

            self.check_merge_commits(),
            self.check_fixup_commits(commits),
            self.check_commit_messages(commits),
            self.check_commit_lengths(commits),
            self.check_author_emails(),
            self.check_submodule_isolation(),
            self.check_submodule_references_exist(),
            self.check_markdown(),