[chore] Tidy up template indentation

[tsmethurst]

Currently our templates are a bit all over the place in terms of how we indent things. This results in HTML which is fine for machines to read, but rather mind-bending for humans to read (see screenshot). It also means we're sending more bytes over the wire than we need to, with all the extra spaces and newlines and whatnot (not a massive issue but still).

We should aim to make the HTML output by the templates more readable and consistent.