Allow for more project-level customization

[amstilp]

This would be a fairly big update that allows a project to customize the behavior of the app more. Some examples of the types of customizations that we'd like to have:

• When a Workspace of a specific type is created, automatically create an auth domain for that workspace with a related name.
• Automatically add a specific group to any ManagedGroup that is created in the app.
• Automatically share a Workspace of a specific type with a specific ManagedGroup.
  ...etc.

This change involves adding new adapter methods (e.g., before_django_create, before_anvil_create, after_django_create, after_anvil_create, etc) and then using them throughout the views or model methods.

[amstilp]

Partially addressed in #499 for workspaces