Subject: Allow configuring the default HEAD branch name for newly-created repositories

All the Create Project interfaces (ssh, REST, web UI) have a default HEAD name of 'master' for the new project.  ssh and REST API allow you to specify branch names, but the web UI seems to not.

The default set of branches for a new project is hardcoded as originating from jgit constants, meaning that maintainers of Gerrit installations can't change the default on their installations.

Can this be made configurable?  Many organizations (e.g. Github) are switching their own default HEAD name for new repositories.  A few of us at the Wikimedia Foundation have been discussing making this change, but discovered that it wasn't trivial to do so.