HTML comments specifying custom templates for page

[simonw]

This would make the custom templating system self-documenting, and save people from having to figure out the right template names for customizing specific pages.

[simonw]

Quoting the new documentation:

You can find out which templates were considered for a specific page by viewing
source on that page and looking for an HTML comment at the bottom. The comment
will look something like this:

<!-- Templates considered: *query-mydb-tz.html, query-mydb.html, query.html -->

This example is from the canned query page for a query called "tz" in the
database called "mydb". The asterisk shows which template was selected - so in
this case, Datasette found a template file called query-mydb-tz.html and
used that - but if that template had not been found, it would have tried for
query-mydb.html or the default query.html.