If JSON() produces UTF-8 bytes, it can't be used in Mason pages with
other content.  Mason pages are constructed using Perl character
strings and then our PSGI response callback in
RT::Interface::Web::Handler encodes all content as UTF-8 if it's not
already encoded.  This leads to double-encoding when JSON() output is
mixed with other content, such as in /Elements/JavascriptConfig.

The autocomplete endpoints which used JSON() worked successfully because
their _entire_ page content was UTF-8 already, so it wasn't encoded
again by the response callback.  By switching JSON() away from UTF-8,
interpolation issues are fixed and the autocomplete endpoints now rely
on the request handler encoding to UTF-8 instead.

Additionally, replace various uses of JSON::to_json() directly with
JSON().