CSS and JS template default value "same as body"

[vincent-peugnet]

I wanted to work on #286, but I felt a bit strange of having three possible value that could be sorted or filtered.

I think that the majority of uses cases of templates is covered by setting BODY, CSS and JS at the same time.

Maybe, the BODY template could be considered as the main template. Not using CSS and JS, or using another template, could be considered as an advanced template option.

This is already the one that is used to determine thumbnail and favicon.

So in the interface, CSS and JS templates could be set by default to "same as BODY" in the page edit view.

Database architecture

1. use the current template value to indicate that it use the same as BODY.
   2.1 storing a special character like * in the CSS/or template value
   2.2 using a null value
   2.3 using a boolean value
2. adding two boolean values called something like templatecsssameasbody

[vincent-peugnet]

Currently, I'm more in favor of the 2.2 design choice

A null could mean "this template setting is not set" ---> "because it use the main setting: BODY template" 😁
And the empty string would mean "do not use any template for CSS/JS".

But maybe I'm exaggerating 😅