Can only specify existing branches for gh pages deployment

When selecting legacy build type with a given deploy branch for GH pages, if the branch does not exist, GitHub will return an error. This is consistent with the Web UI, where you can only select existing branches, however if would be useful if we can even specify non-existant branches as during validation we do know if the branch already exists.

One idea could be that we would create the branch if it does not exist yet, or enhance the validation to retrieve existing branches for the repo.