This is paving the way to move the standard filters into Rust.
Non-empty fields are now determined in Rust, using a single regex
instead of the overkill stripHTMLMedia(). The old implementation
has been moved into the Pystache code for now.
In the process of factoring out the field filtering, the "extra"
and "fullname" args are just passed in as a blank string now.
Extra was functionality that allowed a field modifier to be defined
as "filtername(arg1,arg2):field", and fullname was the name of the
field including any provided field modifiers. From grepping through
the add-ons on AnkiWeb, neither appears to have been used.
