…vior (openedx#62)

Adds support for custom Taxonomy subclasses to be stored against a Taxonomy, to be used by the python API when instantiating and returning Taxonomies.

Also adds minimal support for ObjectTag subclasses. However, these are not stored against the ObjectTag instances; they can be instantiated by the Taxonomy subclasses if and when needed.

Related:
* docs: updates decisions to reflect this change
* feat: adds api.get_taxonomy, which returns a Taxonomy cast to its subclass, when set
* refactor: adds _check_taxonomy, _check_tag, and _check_object methods to the Taxonomy class, which can be overridden by subclasses when validating ObjectTags

Added to support system-defined Taxonomies:
* feat: adds un-editable Taxonomy.system_defined field so that system taxonomies can store this field and ensure no one edits them. 
* feat: adds Taxonomy.visible_to_authors, which is needed for fully automated tagging.

Cleanup changes:
* fix: updates Tag model to cascade delete if the Taxonomy or parent Tag is deleted.
* style: adds missing type annotations to rules and python API

…x#66)

AWS Aurora does not support COMPRESSED in its MySQL implementation:

https://docs.aws.amazon.com/AmazonRDS/latest/AuroraUserGuide/AuroraMySQL.Migrating.RDSMySQL.Import.html