Render partials outside formatting

This fixes a problem @SimonWoolf noticed within DS, whereby the spacing was missing between two blocks of paragraph tags (`<p>...</p>`).
It turns out that this was due to the tags being rendered within a partial, and as such, was getting converted during the compilation of the template (when joining all the partials together), and thus when the final template was formatted, it then stripped the already formatted partials.

Fix is to not format partials, but let Erb combine them into one template first, then just do a one pass format on the whole lot.

@billbillington as well please.
Prefer double-quoted strings unless you need single quotes to avoid extra backslashes for escaping.