New locators for content libraries

This PR implements two new types of locators necessary to facilitate content libraries. I'll be posting a separate PR to edx-platform for the content libraries prototype soon, but for now I'd like to start the review process for this locator code.

The new locators are described in detail under the "New Locators" heading at https://openedx.atlassian.net/wiki/display/SOL/Content+Libraries

Perhaps it'd be better to doctor `_map_revision_to_branch` in `split_draft` to make it easier to implement a review and publishing lifecycle to libraries in the future? That's the fn that maps to split mongo's `default_branch`