Basic asset response support

[ormsbee]

Low-level Learning Core API calls for returning references to content associated with a Component. Uses X-Accel-Redirect.

This adds a number of API calls to the Authoring public API in order to
support associating static assets with Components and allowing them to
be downloaded.

Added:

* get_component_by_uuid
* get_component_version_by_uuid
* get_content_info_headers
* get_redirect_response_for_component_asset

Modified:

* create_next_component_version - made title optional
* create_component_version_content - annotation for learner_downloadable

Most of the justification for this approach can be found in the
docstring for get_redirect_response_for_component_asset.

Note that there is currently no backend redis/memcached caching of the
ComponentVersion and Content information. This means that every time a
request is made, we will do a couple of database queries in order to do
this lookup. I had actually done a version with such caching in an
earlier iteration of this PR, but it added a lot of complexity for what
I thought would be minimal gain, since going through the middleware will
cause about 20 database queries anyway.

This implements the first part of the static assets ADR for Learning
Core: https://github.com/openedx/openedx-learning/pull/110

Important notes:

* No view or auth is implemented. That is the responsibility of
  edx-platform. Learning Core only provides the storage and response
  generation.
* The responses generated will require the use of a reverse proxy that
  can handle the X-Accel-Redirect header. See ADR for details.

[ormsbee]

@bradenmacdonald, @kdmccormick: This is an implementation of static asset serving on the Learning Core side of things. I'm still writing tests and I need to do cleanup on the management command side, but the API and model code should be more or less there. Please feel free to take an early pass at it to make sure it looks sane. Otherwise, I'll ping you folks when it's baked enough for real review.