Add UI.renderWithData; test UI.{insert,render*}
Spacebars: Disallow non-initial `this` in paths

Apparently `../this` used to work as a synonym for `..` in Handlebars, but in Spacebars it actually would look for a property named `this`, which caused confusing breaking changes in apps.
Change {{> content}} to {{> UI.contentBlock}}

...and elseContent to UI.elseBlock.