An openedx_catalog app with a representation for CourseRuns [FC-0117]

[bradenmacdonald]

A very WIP implementation of #469.

Notes

• This adds a new dependency on edx-organizations (which also requires Pillow for the logo field)
• This uses opaque-keys, which was already an indirect dependency but not used directly
• Currently, only course runs have a display_name, and catalog courses do not have names. I'm proposing we add a display_name field to the core CatalogCourse model to support various use cases, including the proposed new studio home page. See the code for how this can be backfilled and how runs can always override the name for each run.

TODOs

• Link platform PR for auto-creating these CourseRuns
• Check if we should deprecate edx-organization's OrganizationCourse

Questions

• There is no stable/portable identifier for CatalogCourse objects, only their internal integer database ID. Do we need this? (An opaque key or something else?)
• Should we make the new CourseRun a SoftDeletableModel to support course deletion without data loss?
• Should we add a 'created' timestamp to the course / catalog course models? (Could use TimeStampedModel but the 'modified' field is rarely going to be relevant, as they're mostly immutable models.)
• Is "Course Language" a core concept that should live in this catalog app? (Presumably at the CatalogCourse level; unless we would ever want to allow different runs of the same catalog course to have different languages?)
• Is "Course Schedule" a core concept that should live in this catalog app? I think yes, but it could also have a draft/publish workflow and ultimately live in openedx_content
• Is "Course Visibility" a core concept that should be in Learning Core? (We actually have it defined as catalog_visibility, visible_to_staff_only, and course_visibility which all have different effects and different enum values, and also need to support "use system default")

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[ormsbee]

There is no stable/portable identifier for CatalogCourse objects, only their internal integer database ID. Do we need this? (An opaque key or something else?)

I don't know if we'll force it to be stable, but we'll probably want a SlugField(allow_unicode=True) so that we can address it in URLs in a descriptive looking way.