themes can have .tpl endings

[fiammybe]

To identify that the templates are not full HTML files, we should be able to use the .tpl ending for the theme templates.
This makes the extension of the file .html.tpl, so we can see what the template is meant to generate :-)

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[github-actions]

This issue is stale because it has been open 60 days with no activity. Remove stale label or comment or assign sombody or this will be closed in 5 days.

[fiammybe]

I tried to make it work with either .html or .html.tpl extensions, but that was adding lots of checks throughout the code. I propose we simply decide on making the switch to .html.tpl extensions at a certain point.