The new SCM API allows:

• Plugins can provide custom categories (e.g. Branches, Pull Requests, Tags, etc)
• Categories are localizable and can have an ordinal value
• SCMSource.getCategories() can be used to fetch the categories for a scm source

In Scope

• We'd like new categories to show up in Blue Ocean in the same way that Branches show up in Blue Ocean today.
• We use the SCMSource's provided names and ordering returned from `getCategories` as tabs in the UI.
• Remove our concept of "branches" and "pull requests"?