Django-markdownx

- Moved html logic from FormField to Widget to be able to override model objects other than included MarkdownxModel
- Fixed default value for `MARKDOWNX_EDITOR_RESIZABLE`

- Warning: no backward compatibility
- Admin, Model and Form custom objects
- Django admin styles for compiled markdown
- Settings variables changed:
- MARKDOWNX_MAX_SIZE => MARKDOWNX_IMAGE_MAX_SIZE
- MARKDOWNX_MARKDOWN_KWARGS => MARKDOWNX_MARKDOWN_EXTENSIONS
- MARKDOWNX_MAX_UPLOADSIZE => MARKDOWNX_UPLOAD_MAX_SIZE
- MARKDOWNX_CONTENT_TYPES => MARKDOWNX_UPLOAD_CONTENT_TYPES