Rethink the default branch name suggestion

[geofflamrock]

Currently we suggest a default branch based on the stack name plus the number of branches e.g. My new feature -> my-new-feature-4.

This is fine but it's hard to tell any context from this branch name when you look at a list of branches, I'm finding it hard to know which branch contained which changes from the name alone. It might be better to suggest some sort of prefix and have the user type in the rest e.g. My new feature -> my-new-feature-{you_fill_in_something_meaningful_here}.