Content libraries settings overrides (SOL-46)

**Background**: This PR allows course authors to use blocks from a content library but override any `Scope.settings` fields such as `display_name` or `weight`. 
**Discussions**: Basic idea was presented on the wiki page for content libraries and discussed w/ a few people at edX
**Dependencies**: Depends on the feature branch https://github.com/edx/edx-platform/commits/content-libraries
**Dependents**: This is currently blocking work on analytics support ( https://github.com/open-craft/edx-platform/pull/26 ), which depends on these changes.
**Sandbox URL**: http://sandbox3.opencraft.com:18010/   (LMS at http://sandbox3.opencraft.com/ but shouldn't be needed)
**Internal code reviews PRs**: https://github.com/open-craft/edx-platform/pull/6 https://github.com/open-craft/edx-platform/pull/25
**Partner information**: 3rd party-hosted open edX instance, for an edX solutions client.

**Author notes**: Capa modules unfortunately store some of their *content* ("markdown") as `Scope.settings`, which throws a wrench into the works. For this PR I have added a capa-specific workaround (20d07d210074081a038fec781e484539f1022b31).