Fix EZP-22858: Display in HTML Comments name of the used template

> https://jira.ez.no/browse/EZP-22858
> Replaces #860 

This PR is an enhancement of #860.
It ensures that start/stop template comments are correctly displayed for layout templates, to avoid quirks with some (old) browsers.

1. Start comment **after** DocType declaration
2. Stop comment **before** `</body>`

**This only affects HTML templates**