When `HX-Target` header is present, bypass any configured layout for the
request. This allows a full page reload to render with a layout, i.e.
render the entire page, while a request from htmx will load just the
content directly generated by the view.
Latest Zmpl provides `template.renderWithLayout(other_template, data)`,
allowing a template to be renedered within another template.
Create layouts in `src/app/views/layouts/` or use
`jetzig generate layout [name]` and set `pub const layout = "name";` in
each view file.
