Edited cms models and updated tests

- Added `content` to Page
- Added `is_published` to Page
- Added `get_page` static method to Page
- Renamed `PageSection.section_type` to `PageSection.type`
- Added extra relationships for PageSection subtypes
- Removed `TextSection.validate_content`, html escaping should be done
in the view
Wrote part of views for editing page

Not everything works yet because the template isn't fully designed.
Added TextSection creation to add_debug_data
Copied slugify function from the internet

I'm not using it yet, maybe I will never.