Use underscore in {title} for branch name

[daveisfera]

We use a pattern of {id}-{title} (which I believe is fairly common) so that GitLab and Jira will automatically know which card the MR is for, but it puts dash (-) in the expansion of {title} and it is a bit more readable when underscore (_) is used so the division between the {id} and {title} is very apparent