Currently, if you want to change a single template, you have to copy all of the templates to a custom location, and set email.template_dir. That's a nuisance, and it means that whenever we introduce a new template (as per Synapse 1.4.0), people have to make sure they manually install the new template.

It also repeatedly confuses people who think that they need to set template_dir and do so without realising the implications.

(Also: hiding the template_dir setting under email when it is used for non-email things is very silly.)

A better approach would be to look in the template_dir, and fall back to the default templates if the one we're looking for is missing.