Make built-in models abstract

If we want to continue using CODERED_FRONTEND_* settings (which I think we do, they will be the foundation of themes), we will have to make everything that uses our streamfield blocks abstract.

The issue is, many coderedcms models use streamfield blocks that pull values from CODERED_FRONTEND_* settings. Which means if you change one of these settings in your project, it triggers a migration in coderedcms. Which is a bad practice and leads to endless migrations.

This fits in with the paradigm we have been following with the page types. Provide beefy abstract models, and provide a very basic concrete model boilerplate in the project template.

One caveat to making the snippets abstract, is that we will no longer be able to provide a snippet chooser, or hard-code templates that pull in navbar/footer snippets. So this will have to be thought through to figure out how to gracefully handle all of this without re-defining everything in the project template (which would be too much boilerplate I think).

Anyhow, this pretty major regarding future stability. Implementing it will inevitably be a breaking change. But there is no other way to have dynamic blocks and themes. So I think it is necessary.

Bắt đầu bằng cách lập bản đồ các model tích hợp sẵn, các block StreamField, các setting CODERED_FRONTEND_*, cách xử lý snippet và project template được đề cập trong issue. Xác định các model trừu tượng sẽ ảnh hưởng như thế nào đến việc chọn snippet, các template navbar/footer và migration; công việc được xem là hoàn tất khi có một thiết kế được ghi lại, tránh các migration setting dành riêng cho project mà không cần quá nhiều boilerplate.