Add collapsible attribute to lists

[newbthenewbd]

It doesn't occur to me that collapsing a list with a single field in it has any use aside from looking ugly. Fieldsets already have this, too, so here we go. The attribute defaults to true, so both backward and forward compatibility should be preserved just fine. (well, if an older grav-plugin-admin version tried to load a blueprint with this attribute set to 0, it would just be ignored, I suppose that's nice enough compatibility)

[w00fz]

This was not going to work, the check should have been field.collapsible is not defined or field.collapsible instead of if field.collapsible is empty or field.collapsible.

I updated it for you. I like the idea and consistency this change makes with fieldsets, thanks!

[newbthenewbd]

Thank You for the update and merging!

In my tests, is empty worked, but I sure do not have enough expertise with Twig to rate as to which way is the correct one. After the update, however, there is still a single is empty check at line 68 of list.html.twig. Is that intentional?