Fix external JS template to work with the TW5 webserver

It's a bit gross that we have to change the filename used to reference the JS file. This is to make it work with the webserver.

At the moment, the webserver exposes system tiddlers as plain text renderings, and ordinary tiddlers as full HTML renderings through a view template. So we have to use a system tiddler title for the JS file.

The workaround I'm thinking of is to remove the blanket exposure of system tiddlers, and instead have a list of system tiddlers that are specifically exposed through a namespace like `127.0.0.1:8080\lib\tiddlywiki.js`. That can't clash with a tiddler title because tiddler titles are URI encoded and so can't contain slashes.

core/templates/external-js/tiddlywiki5-external-js.html.tid

@@ -37,5 +37,5 @@ title: $:/core/templates/tiddlywiki5-external-js.html
 <!--~~ Ordinary tiddlers ~~-->
 {{$:/core/templates/store.area.template.html}}
 </body>
-<script src="tiddlywiki.js" onerror="alert('Error: Cannot load tiddlywiki.js');"></script>
+<script src="%24%3A%2Fcore%2Ftemplates%2Ftiddlywiki5.js" onerror="alert('Error: Cannot load tiddlywiki.js');"></script>
 </html>